Try this. Make a new save with just elk mountain and UnitConvert. See if it works with just those things. If it does, start adding mods one or a few at a time until the error happens. Focus on mods that add new fillTypes. In other words, ignore vehicles and such as that is most likely not the problem.
If it errors with JUST Elk Mountain, I'll have to see why. The idea is to narrow down and find the offender. I haven't had a chance to play FS22 yet, so I don't have many mods. Only the ones people tend to have problems with. Production Revamp COULD cause and issue and currently DOES cause an issue with Animal Production, so it may affect UnitConvert as well. It's written a little more "brute force" than a lot of mods, making it hard to work around. Add that one to the list of mods to try on your new save, even though I don't think it adds any fillTypes.
If you can't find the conflicting mod, send me a message at m.me/ThundRFSMods or go to my facebook site and message me from there. I'll help you figure out what's going on. The error is a misconfigured or late registered fillType. In order to work around it (until the new version is ready), I have to know what fillType(s) are causing the issue.