In reply to carstorm:
One thing I do is I export the instance, check overrides folder, then check mods folder. Make sure everything in that folder is on the 3rd party mod list. If its not, it will be rejected. It just lets you preemptively check if you have anything that needs approved outside of direct mod downloads. Ive caught a few mod updates that way as well. Sometimes old versions of mods get removed from the site and end up in the overrides folder.