I'm trying to start a world using Entopy mod and other mods, as well as with a randomizer datapack, and the game refuses to start the world with the error

Encountered exception while handling in channel with name "entropy:join-handshake"
java.lang.NullPointerException: Cannot read field "currentEvents" because "this.eventHandler" is null

latest.log and regular log (Gist)

This is the line that's crashing, because in line 87 the check failed as there were not exactly one players on the server (either zero or at least 2). I wonder if there is a reason the check isn't just a simple null check.

Good news, with only Entropy and Fabric API it loaded a world perfectly, and I saw the timer bar going, so one of the mods I have is causing the issue.

Do you have any idea how I could solve this, or is it an issue in the mod itself ?

Since this is singleplayer, I only have the idea of removing mods until the world starts. There might be a mod that causes that code to run too early. The problem is that Entropy seems to only initialize when exactly one player is present. Otherwise it'll crash like you're experiencing.

I'll try to start with only fabric api and entropy and see if I get the issue again when I get home in a bit. I'll keep you updated

Okay, so the mod that was causing problems was the helper mod from a Minecraft Randomizer (https://fasguy.net/minecraft_toolbox/loot-table-randomizer). Now the mod works perfectly, thank you for your help.