After entering the user and pressing "Play Demo" from the launcher, infinitely "Preparing..." is displayed. Here is the log:
[21:01:11 INFO]: Getting syncinfo for selected version
[21:01:11 INFO]: Queueing library & version downloads
[21:01:11 FATAL]: Unhandled exception in thread Thread[pool-2-thread-2,5,main]
com.google.gson.JsonSyntaxException: duplicate key: null
at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:191) ~[minecraft.jar:?]
at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:146) ~[minecraft.jar:?]
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:93) ~[minecraft.jar:?]
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:172) ~[minecraft.jar:?]
at com.google.gson.Gson.fromJson(Gson.java:795) ~[minecraft.jar:?]
at com.google.gson.Gson.fromJson(Gson.java:761) ~[minecraft.jar:?]
at com.google.gson.Gson.fromJson(Gson.java:710) ~[minecraft.jar:?]
at com.google.gson.Gson.fromJson(Gson.java:682) ~[minecraft.jar:?]
at net.minecraft.launcher.updater.RemoteVersionList.getCompleteVersion(RemoteVersionList.java:32) ~[launcher.jar:1.6.89-j]
at net.minecraft.launcher.updater.MinecraftVersionManager.getLatestCompleteVersion(MinecraftVersionManager.java:213) ~[launcher.jar:1.6.89-j]
at net.minecraft.launcher.updater.MinecraftVersionManager.getLatestCompleteVersion(MinecraftVersionManager.java:37) ~[launcher.jar:1.6.89-j]
at com.mojang.launcher.game.runner.AbstractGameRunner.playGame(AbstractGameRunner.java:67) ~[launcher.jar:1.6.89-j]
at net.minecraft.launcher.game.GameLaunchDispatcher$1.run(GameLaunchDispatcher.java:111) ~[launcher.jar:1.6.89-j]
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) ~[?:?]
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) ~[?:?]
at java.base/java.lang.Thread.run(Thread.java:834) [?:?]
I also tried Oracle Java 15 from PPA and run sudo update-alternatives --config java, so the launcher wouldn't start.
Openjdk-8-jre or changing the profile version 1.16.x or 1.15.x also didn't work at all. The Log:
[10:01:16 INFO]: Logging in with access token
[10:01:26 INFO]: Getting syncinfo for selected version
[10:01:26 INFO]: Queueing library & version downloads
[10:01:26 FATAL]: Unhandled exception in thread Thread[pool-2-thread-2,5,main]
com.google.gson.JsonSyntaxException: duplicate key: null
at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:191) ~[minecraft.jar:?]
at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:146) ~[minecraft.jar:?]
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:93) ~[minecraft.jar:?]
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:172) ~[minecraft.jar:?]
at com.google.gson.Gson.fromJson(Gson.java:795) ~[minecraft.jar:?]
at com.google.gson.Gson.fromJson(Gson.java:761) ~[minecraft.jar:?]
at com.google.gson.Gson.fromJson(Gson.java:710) ~[minecraft.jar:?]
at com.google.gson.Gson.fromJson(Gson.java:682) ~[minecraft.jar:?]
at net.minecraft.launcher.updater.RemoteVersionList.getCompleteVersion(RemoteVersionList.java:32) ~[launcher.jar:1.6.89-j]
at net.minecraft.launcher.updater.MinecraftVersionManager.getLatestCompleteVersion(MinecraftVersionManager.java:213) ~[launcher.jar:1.6.89-j]
at net.minecraft.launcher.updater.MinecraftVersionManager.getLatestCompleteVersion(MinecraftVersionManager.java:37) ~[launcher.jar:1.6.89-j]
at com.mojang.launcher.game.runner.AbstractGameRunner.playGame(AbstractGameRunner.java:67) ~[launcher.jar:1.6.89-j]
at net.minecraft.launcher.game.GameLaunchDispatcher$1.run(GameLaunchDispatcher.java:111) ~[launcher.jar:1.6.89-j]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) ~[?:1.8.0_275]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) ~[?:1.8.0_275]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_275]