Description: Exception in server tick loop

java.lang.IllegalStateException: Not a JSON Object: "\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000"
at Not Enough Crashes deobfuscated stack trace.(1.17.1+build.61)
at com.google.gson.JsonElement.getAsJsonObject(JsonElement.java:90)
at hardcorequesting.common.fabric.quests.QuestingDataManager$State.loadFromString(QuestingDataManager.java:129)
at java.util.Optional.ifPresent(Optional.java:178)
at hardcorequesting.common.fabric.quests.SimpleSerializable.load(SimpleSerializable.java:36)
at hardcorequesting.common.fabric.quests.QuestLine.loadAll(QuestLine.java:183)
at hardcorequesting.common.fabric.event.WorldEventListener.onLoad(WorldEventListener.java:18)
at me.shedaniel.cloth.api.common.events.v1.WorldLoadCallback.lambda$static$0(WorldLoadCallback.java:39)
at net.minecraft.server.world.ServerWorld.handler$zoc000$init(ServerWorld:4594)
at net.minecraft.server.world.ServerWorld.(ServerWorld:224)
at net.minecraft.server.MinecraftServer.createWorlds(MinecraftServer:383)
at net.minecraft.server.MinecraftServer.loadWorld(MinecraftServer:350)
at net.minecraft.server.dedicated.MinecraftDedicatedServer.setupServer(MinecraftDedicatedServer:184)
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer:670)
at net.minecraft.server.MinecraftServer.method_29739(MinecraftServer:270)
at java.lang.Thread.run(Thread.java:831)

Assuming it's not done already, we should probably be catching the exception and rethrowing it with the name of the affected file in the error message

the thing is, that i deleted the config file to test again.
it works on singleplayer, but still crashes on server. Seems like it's not a problem with my quests.

Your HQM quest files are invalid, like the actuall text in these files doesn't fit to the standard HQM needs to read it.
Please make sure all the quest files are valid .json format. There are tester on the internet, where you can check them easily.

In particular, it is the state.json file that it has trouble reading

This has to do with the data file state.json located in the world folder, and not any of the config files.

okay, where can i verify my quests ?

https://jsonlint.com/ should work for what canitzp described.
But again, it is primarily state.json in the world folder that you should be looking at.