Make sure that you're installing mods to the correct location, and deleting them from the correct location.
That'd be Documents/mygames/Cosmoteer/Mods. While there is a mod folder in the game data files that's for the built in mods. Installing them to the correct location should prevent any future issues.
As for fixing the issue you can make sure you're running the game without mods by either using the searching the start menu for "Cosmoteer (No mods)" and running that, or going to Documents/mygames/Cosmoteer and opening settings.txt then removing the mods from it.
If neither of those work you can try reinstalling the game.
Sorry I don't have a better solution but I've never seen that error before. If you could go to Documents/mygames/Cosmoteer/Logs open the most recent one, copy it over to Pastebin, then share the pastebin link here that might give us some more info.
