I'm playing a custom 1.20.1 modpack and everytime I try to open an old world it shows a screen that says, "Errors in currently selected data packs prevented the world from loading. You can either try to load it with only the vanilla data pack ("safe mode"), or go back to the title screen and fix it manually."

Pressing Safe Mode leads to a screen that says, "Failed to load world in Safe Mode. This world contains invalid or corrupted save data." I have tried making new worlds and it's always the same, I'm able to get into the world the first time then can't rejoin it.

Here is a log from when I tried to open the world, https://pastebin.com/9wAvHWwL

And this is the entire latest log, https://mclo.gs/qkf06Ns

Typically—at least in my case—the broken datapack thing always turns out to be a broken mod. Someone else here might have a better answer for you than I do, but when that happens, I go through all my mods, disabling them one by one until I locate the culprit. It’s a headache, but it’s the only way I know how to do it.