Thanks for this answer. Since I need Java 11 for other purposes on my computer, is there a way to tell Minecraft which Java version to use?
The launcher seems to have this option, but I don't know what to put in the field for the java executable. Currently I have Java 8 and Java 11 installed as follows:
java-1.11.0-openjdk-amd64 1111 /usr/lib/jvm/java-1.11.0-openjdk-amd64
java-1.8.0-openjdk-amd64 1081 /usr/lib/jvm/java-1.8.0-openjdk-amd64
Specifying the path /usr/lib/jvm/java-1.8.0-openjdk-amd64 in the launcher field for the forge installation produces a rather confusing error message:
Unable lo locate the Java runtime
Error details: Success
Filename on disk: java-8-openjdk-amd64
Path: /usr/lib/jvm/java-8-openjdk-amd64
Exists: Directory
Also, the vanilla versions of Minecraft seem to work perfectly fine with the Java 11 (it is just the Forge versions that produce the error).