All versions up to 1.14.4 (including pre7, but not the release itself) works fine. The only snapshots that work is 19w34a and 19w35a.

1.14.4 used to work until week 36, but stopped working after that. The snapshots after week 35 have never worked.

The following versions are affected:

1.14.4

snapshot 19w36a

snapshot 19w37a

snapshot 19w38a

snapshot 19w38b

snapshot 19w39a

 I have tried:

• restarting computer
• restarting launcher
• searching for solution on google in in Jira (found many similar issues, but none with the exact same exception and affecting the exact same versions)

Is there anything else I could try?

Here's the output from the terminal:

$ Minecraft/start-client
Bootstrap (v5)
Current time is Sep 29, 2019 1:22:35 PM
System.getProperty('os.name') == 'Linux'
System.getProperty('os.version') == '4.4.0-164-generic'
System.getProperty('os.arch') == 'amd64'
System.getProperty('java.version') == '1.8.0_144'
System.getProperty('java.vendor') == 'Oracle Corporation'
System.getProperty('sun.arch.data.model') == '64'

Looking for update
Downloading: https://s3.amazonaws.com/Minecraft.Download/launcher/launcher.pack.lzma
Got reply in: 510ms
No update found.
Reversing LZMA on /home/ton/.minecraft/launcher.pack.lzma to /home/ton/.minecraft/launcher.pack
Unpacking /home/ton/.minecraft/launcher.pack to /home/ton/.minecraft/launcher.jar
Cleaning up /home/ton/.minecraft/launcher.pack
Starting launcher.
[13:22:40 INFO]: Minecraft Launcher 1.6.89-j (through bootstrap 5) started on linux...
[13:22:40 INFO]: Current time is Sep 29, 2019 1:22:40 PM
[13:22:40 INFO]: System.getProperty('os.name') == 'Linux'
[13:22:40 INFO]: System.getProperty('os.version') == '4.4.0-164-generic'
[13:22:40 INFO]: System.getProperty('os.arch') == 'amd64'
[13:22:40 INFO]: System.getProperty('java.version') == '1.8.0_144'
[13:22:40 INFO]: System.getProperty('java.vendor') == 'Oracle Corporation'
[13:22:40 INFO]: System.getProperty('sun.arch.data.model') == '64'
[13:22:40 INFO]: proxy == DIRECT
[13:22:40 INFO]: JFX is already initialized
[13:22:40 INFO]: Refreshing local version list...
[13:22:40 INFO]: Refreshing remote version list...
[13:22:41 INFO]: Refresh complete.
[13:22:41 INFO]: Loaded 7 profile(s); selected 'Anthony'
[13:22:41 INFO]: Refreshing auth...
[13:22:41 INFO]: Logging in with access token
[13:22:53 INFO]: Getting syncinfo for selected version
[13:22:53 INFO]: Queueing library & version downloads
[13:22:53 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$1.run(GameLaunchDispatcher.java:111) ~[launcher.jar:1.6.89-j]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) ~[?:1.8.0_144]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) ~[?:1.8.0_144]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_144]