If an unknown mod is in the list of the ExcludeMods setting, a Minecraft Forge 1.15.2-31.2.36 server will fail to start. Removing mods from this list that are not installed resolves the issue.

Never seen it happen and no further details given, also the workaround is to remove entries for mods not loaded, so closing this issue.

I created a server with that version and it starts fine. I also connected to it without a problem. I saw the Unknown mods reported in the log file as they should be.
Can you provide some more info?

I can try to get you a stack trace if you want, but it was rather non-descript.

Sure anything that can help