The server is waiting for a registry decision
Applies to Forge, NeoForge.
What it looks like in your log
Run the command /fml confirm or /fml cancel to proceed. Alternatively start the server with -Dfml.queryResult=confirm
What causes it
Forge found world content it does not recognise and is waiting for confirmation before erasing it. The server will not finish starting until answered.
How to fix it
Back the world up first, then run /fml confirm in the console to continue. Reinstall the missing mods instead if you want to keep that content.
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