The cause is likely that your constants mod version does not match the one packaged with the game, because it is stored separately from the game data. The game is unable to handle this properly.
First, I recommend resetting your mods, may not be necessary but it's a good precaution.
Then, find and delete the constants mod folder at C:\Users\USERNAME\AppData\Roaming\Strive\mods\Constants.
Finally, a new constants folder will be copied over when the game starts, use that when re-applying your mods.