Investigated further: I can confirm it does not work also on Windows. The log below is from a Windows machine using the module of Memoir '44. Could someone else please confirm that they are not able to create a New Extension? You can test it in a few seconds. Just open vassal, right-click on a module in the list of the installed modules, and select ‘New Extension’. A newly created extension should appear, but I do not get anything.
2021-02-04 00:23:26,198 [15520-main] INFO VASSAL.launch.StartUp - Starting
2021-02-04 00:23:26,209 [15520-main] INFO VASSAL.launch.StartUp - OS Windows 10 10.0
2021-02-04 00:23:26,209 [15520-main] INFO VASSAL.launch.StartUp - Java version 15.0.1
2021-02-04 00:23:26,209 [15520-main] INFO VASSAL.launch.StartUp - VASSAL version 3.5.0
2021-02-04 00:23:26,278 [15520-AWT-EventQueue-0] INFO VASSAL.launch.ModuleManager - Manager
2021-02-04 00:23:33,038 [15520-SwingWorker-pool-2-thread-1] INFO VASSAL.launch.AbstractLaunchAction - Loading module file C:\Users\giuli\OneDrive\Desktop\Modules\Memoir44vX-10.14.vmod
2021-02-04 00:23:33,225 [15520-SwingWorker-pool-2-thread-1] INFO VASSAL.launch.TilingHandler - No images to tile.
2021-02-04 00:23:33,227 [15520-SwingWorker-pool-2-thread-1] INFO VASSAL.launch.AbstractLaunchAction - Loading module Memoir '44
2021-02-04 00:23:33,229 [15520-SwingWorker-pool-2-thread-1] INFO VASSAL.tools.io.ProcessLauncher - launching C:\Program Files\VASSAL-3.5.0\jre\bin\java -Xms512M -Xmx512M -Duser.home=C:\Users\giuli -Duser.dir=C:\Program Files\VASSAL-3.5.0 -cp lib\Vengine.jar VASSAL.launch.Editor --new-extension -- C:\Users\giuli\OneDrive\Desktop\Modules\Memoir44vX-10.14.vmod
2021-02-04 00:23:33,657 [9752-main] INFO VASSAL.launch.StartUp - Starting
2021-02-04 00:23:33,665 [9752-main] INFO VASSAL.launch.StartUp - OS Windows 10 10.0
2021-02-04 00:23:33,666 [9752-main] INFO VASSAL.launch.StartUp - Java version 15.0.1
2021-02-04 00:23:33,666 [9752-main] INFO VASSAL.launch.StartUp - VASSAL version 3.5.0
2021-02-04 00:23:33,666 [9752-main] INFO VASSAL.launch.Launcher - Editor
2021-02-04 00:23:34,327 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: GameState.load_continuation.shortcut
2021-02-04 00:23:34,703 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,715 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,743 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,748 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,759 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,764 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,799 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,802 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,808 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,811 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,813 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,815 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,818 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,821 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,835 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,838 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,839 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,841 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,843 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,844 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,850 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,851 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,853 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,854 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,856 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,858 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,860 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,862 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,863 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,865 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,867 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,869 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,870 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,872 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,874 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,876 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,879 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,882 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,898 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,900 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,900 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,902 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,903 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,905 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,906 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,908 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,909 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:34,911 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:35,531 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:35,532 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:35,534 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:35,535 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:36,096 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:36,129 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:36,133 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:36,160 [9752-AWT-EventQueue-0] WARN VASSAL.i18n.BundleHelper - No Translation: Deck.deck
2021-02-04 00:23:36,170 [9752-AWT-EventQueue-0] INFO VASSAL.build.GameModule - Memoir '44 version 10.14
2021-02-04 00:23:36,193 [9752-AWT-EventQueue-0] ERROR VASSAL.build.module.ModuleExtension - Not an extension file C:\Users\giuli\AppData\Local\Temp\vassal_10595153591519057632\tmp_13460958355371515983.vext
2021-02-04 00:23:36,197 [9752-AWT-EventQueue-0] ERROR VASSAL.tools.ProblemDialog -
VASSAL.build.module.ExtensionsLoader$LoadExtensionException: Not an extension file C:\Users\giuli\AppData\Local\Temp\vassal_10595153591519057632\tmp_13460958355371515983.vext
at VASSAL.build.module.ModuleExtension.build(ModuleExtension.java:126)
at VASSAL.launch.NewExtensionAction.performAction(NewExtensionAction.java:45)
at VASSAL.launch.Editor.launch(Editor.java:90)
at VASSAL.launch.Launcher$1.run(Launcher.java:86)
at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:316)
at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:770)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:740)
at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)