diff --git a/.github/workflows/nightly-artifacts.yml b/.github/workflows/nightly-artifacts.yml
index 2bc286e..4809147 100644
--- a/.github/workflows/nightly-artifacts.yml
+++ b/.github/workflows/nightly-artifacts.yml
@@ -71,7 +71,7 @@ jobs:
             --main-jar app.jar `
             --main-class kst4contest.view.Kst4ContestApplication `
             --module-path target/dist-libs `
-            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec `
+            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net `
             --dest dist
 
       - name: Create Windows ZIP
@@ -128,7 +128,7 @@ jobs:
             --main-jar app.jar \
             --main-class kst4contest.view.Kst4ContestApplication \
             --module-path target/dist-libs \
-            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec \
+            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net \
             --dest dist
 
       - name: Create AppDir metadata
@@ -214,7 +214,7 @@ jobs:
             --main-jar app.jar \
             --main-class kst4contest.view.Kst4ContestApplication \
             --module-path target/dist-libs \
-            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec \
+            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net \
             --linux-package-deps "libgstreamer1.0-0,libgstreamer-plugins-base1.0-0,gstreamer1.0-plugins-good" \
             --dest dist
           DEB="$(ls dist/*.deb | head -n 1)"
@@ -274,7 +274,7 @@ jobs:
             --main-jar app.jar \
             --main-class kst4contest.view.Kst4ContestApplication \
             --module-path target/dist-libs \
-            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec \
+            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net \
             --linux-package-deps "gstreamer1,gstreamer1-plugins-base,gstreamer1-plugins-good" \
             --dest dist
           RPM="$(ls dist/*.rpm | head -n 1)"
@@ -336,7 +336,7 @@ jobs:
             --main-jar app.jar \
             --main-class kst4contest.view.Kst4ContestApplication \
             --module-path target/dist-libs \
-            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec \
+            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net \
             --dest dist
 
       - name: Build Arch Linux package artifact
@@ -455,7 +455,7 @@ jobs:
             --main-jar app.jar \
             --main-class kst4contest.view.Kst4ContestApplication \
             --module-path target/dist-libs \
-            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec \
+            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net \
             --dest target/flatpak-src
 
       - name: Create Flatpak manifest
@@ -643,7 +643,7 @@ jobs:
             --main-jar app.jar \
             --main-class kst4contest.view.Kst4ContestApplication \
             --module-path target/dist-libs \
-            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec \
+            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net \
             --dest dist
 
         env:
diff --git a/.github/workflows/tagged-release.yml b/.github/workflows/tagged-release.yml
index cba6638..5e0b82e 100644
--- a/.github/workflows/tagged-release.yml
+++ b/.github/workflows/tagged-release.yml
@@ -56,7 +56,7 @@ jobs:
             --main-jar app.jar `
             --main-class kst4contest.view.Kst4ContestApplication `
             --module-path target/dist-libs `
-            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec `
+            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net `
             --dest dist
 
       - name: Create Windows ZIP
@@ -106,7 +106,7 @@ jobs:
             --main-jar app.jar \
             --main-class kst4contest.view.Kst4ContestApplication \
             --module-path target/dist-libs \
-            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec \
+            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net \
             --dest dist
 
       - name: Create AppDir metadata
@@ -185,7 +185,7 @@ jobs:
             --main-jar app.jar \
             --main-class kst4contest.view.Kst4ContestApplication \
             --module-path target/dist-libs \
-            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec \
+            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net \
             --linux-package-deps "libgstreamer1.0-0,libgstreamer-plugins-base1.0-0,gstreamer1.0-plugins-good" \
             --dest dist
           DEB="$(ls dist/*.deb | head -n 1)"
@@ -238,7 +238,7 @@ jobs:
             --main-jar app.jar \
             --main-class kst4contest.view.Kst4ContestApplication \
             --module-path target/dist-libs \
-            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec \
+            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net \
             --linux-package-deps "gstreamer1,gstreamer1-plugins-base,gstreamer1-plugins-good" \
             --dest dist
           RPM="$(ls dist/*.rpm | head -n 1)"
@@ -291,7 +291,7 @@ jobs:
             --main-jar app.jar \
             --main-class kst4contest.view.Kst4ContestApplication \
             --module-path target/dist-libs \
-            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec \
+            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net \
             --dest dist
 
       - name: Build Arch Linux package artifact
@@ -401,7 +401,7 @@ jobs:
             --main-jar app.jar \
             --main-class kst4contest.view.Kst4ContestApplication \
             --module-path target/dist-libs \
-            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec \
+            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net \
             --dest target/flatpak-src
 
       - name: Create Flatpak manifest
@@ -531,7 +531,7 @@ jobs:
             --main-jar app.jar \
             --main-class kst4contest.view.Kst4ContestApplication \
             --module-path target/dist-libs \
-            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec \
+            --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net \
             --dest dist
 
         env:
diff --git a/packaging/aur/kst4contest-git/PKGBUILD b/packaging/aur/kst4contest-git/PKGBUILD
index 937106b..ca3fb74 100644
--- a/packaging/aur/kst4contest-git/PKGBUILD
+++ b/packaging/aur/kst4contest-git/PKGBUILD
@@ -40,7 +40,7 @@ build() {
         --main-jar app.jar \
         --main-class kst4contest.view.Kst4ContestApplication \
         --module-path target/dist-libs \
-        --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec \
+        --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net \
         --dest dist
 }
 
diff --git a/packaging/aur/kst4contest/PKGBUILD b/packaging/aur/kst4contest/PKGBUILD
index 6c7b2b8..467b08b 100644
--- a/packaging/aur/kst4contest/PKGBUILD
+++ b/packaging/aur/kst4contest/PKGBUILD
@@ -33,7 +33,7 @@ build() {
         --main-jar app.jar \
         --main-class kst4contest.view.Kst4ContestApplication \
         --module-path target/dist-libs \
-        --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec \
+        --add-modules javafx.controls,javafx.graphics,javafx.fxml,javafx.web,javafx.media,java.sql,java.net.http,jdk.crypto.ec,jdk.net \
         --dest dist
 }
 
diff --git a/src/main/java/kst4contest/controller/On4KstConnectionManager.java b/src/main/java/kst4contest/controller/On4KstConnectionManager.java
index ecbac75..ae74860 100644
--- a/src/main/java/kst4contest/controller/On4KstConnectionManager.java
+++ b/src/main/java/kst4contest/controller/On4KstConnectionManager.java
@@ -285,7 +285,10 @@ final class On4KstConnectionManager {
             scheduler.schedule(
                     () -> sendLogin(token), LOGIN_FALLBACK_MILLIS,
                     TimeUnit.MILLISECONDS);
-        } catch (Exception exception) {
+        } catch (Throwable exception) {
+            // Errors must be caught as well: an Error escaping here would be
+            // swallowed by the scheduler and leave the state machine stuck in
+            // CONNECTING without any reconnect attempt or user visible failure.
             try {
                 socket.close();
             } catch (IOException ignored) {
@@ -699,7 +702,9 @@ final class On4KstConnectionManager {
             socket.setOption(ExtendedSocketOptions.TCP_KEEPIDLE, 45);
             socket.setOption(ExtendedSocketOptions.TCP_KEEPINTERVAL, 15);
             socket.setOption(ExtendedSocketOptions.TCP_KEEPCOUNT, 3);
-        } catch (UnsupportedOperationException | IOException exception) {
+        } catch (UnsupportedOperationException | IOException | LinkageError exception) {
+            // LinkageError covers runtime images built without the jdk.net module;
+            // the connection stays usable, only kernel side keepalive is missing.
             LOGGER.log(Level.INFO,
                     "Platform does not support configurable TCP keepalive; "
                             + "application heartbeat remains active", exception);