The world remembers a mod that is gone
Applies to Forge, NeoForge.
What it looks like in your log
This world was saved with mod create which appears to be missing, things may not work well
What causes it
This world was last saved with a mod that is no longer installed. Its content may vanish or misbehave.
How to fix it
Reinstall the named mod to keep its content, or continue knowingly after a backup.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log