I have problem that I accidentally installed the mod thinking that it does something else. The problem is that now I can not play my world without the mod. Its problem also that I can not jump to 1.21 without the mod. From my last save without the mod its more then 100h and this is my main world for many years. Is there any chance that I can free my save?

Thanks a lot. Idk how to restart the level.dat as I only have SP world, but replacing file level.dat with file from another world fixed the issue. (only problem is that my e-chest content got erased).

You may be encountering the same issue as #249.

Without logs, I cannot thoroughly diagnose the issue.

Potential Fix
Try resetting your server's worldgen NBT data (level.dat) to default.