I can't start Minecraft (1.14.4) launcher 1.6.89-j.
I'm running it on Xubuntu 16.04.6 LTS -the system is updated.

(By the way: I had problems uploading this issue because bugs.mojang said

"versions: Eine Version mit ID 'Minecraft (1.14.4) launcher 1.6.89-j' existiert nicht."

versions are not existing, but yes here they are running well till last week, I had to select another (wrong) version to be able to upload the issue)

First I had following problem:
"Error message after starting minecraft launcher (Game Output)
OpenJDK 64-Bit Server VM warning: Using incremental CMS is deprecated and will likely be removed in a future release"
"Fehler: Hauptklasse net.minecraft.client.main.Main konnte nicht gefunden oder geladen werden."

Not working.
And today I have something probably related to https://bugs.mojang.com/projects/MCL/issues/MCL-11992?filter=allopenissues
The launcher stops at "PREPARING..."
"[10:24:46 FATAL]: Unhandled exception in thread Thread[pool-2-thread-2,5,main]"

My whole logfile :

[10:24:43 INFO]: Minecraft Launcher 1.6.89-j (through bootstrap 5) started on linux...
[10:24:43 INFO]: Current time is Sep 6, 2019 10:24:43 AM
[10:24:43 INFO]: System.getProperty('os.name') == 'Linux'
[10:24:43 INFO]: System.getProperty('os.version') == '4.15.0-60-generic'
[10:24:43 INFO]: System.getProperty('os.arch') == 'amd64'
[10:24:43 INFO]: System.getProperty('java.version') == '1.8.0_222'
[10:24:43 INFO]: System.getProperty('java.vendor') == 'Private Build'
[10:24:43 INFO]: System.getProperty('sun.arch.data.model') == '64'
[10:24:43 INFO]: proxy == DIRECT
[10:24:44 INFO]: Refreshing local version list...
[10:24:44 INFO]: Refreshing remote version list...
[10:24:45 INFO]: Refresh complete.
[10:24:45 INFO]: Loaded 1 profile(s); selected 'ballohallo'
[10:24:45 INFO]: Refreshing auth...
[10:24:45 INFO]: Logging in with access token
[10:24:46 INFO]: Getting syncinfo for selected version
[10:24:46 INFO]: Queueing library & version downloads
[10:24:46 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$1.run(GameLaunchDispatcher.java:111) ~[launcher.jar:1.6.89-j]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) ~[?:1.8.0_222]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) ~[?:1.8.0_222]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_222]

Thank you for the help