See image for error message:
https://ibb.co/B3BfNJL

All I did was change some of the XP variables to try and make leveling up progress faster, and now any time I gain xp after the first time, it breaks.

I think I maybe know what happened. The multipliers etc at the bottom aren't very clear what they do/how they affect over-all exp gain... I had exp multiplier set to 100, and maybe, just maybe, the sheer amount of exp is going over the maximum and back into the negatives, and that breaks it?

I dunno just guessing.

To help fix any issues, is there a way to reset the mod settings to default, so I can change things one at a time to see what they do?

I've noticed that completely removing the mod and reinstalling it does not change the settings back to default, it still keeps the settings I had entered in the main menu.