Turns out that when teambuilding your nickname is empty when you validate, so the validator thinks you're not cross evolving, but when you get to the battle your nickname becomes your species, however the code wasn't expecting that and tried to cross evolve anyway. This is now fixed.
While I'm here, the bug with the types that I thought I had fixed yesterday still wasn't fixed. Turns out that there was a very similar bug on main which started affecting my server when I updated it last week. The bug has since been patched on main too. (The bug only affects certain OMs such as Reliablemons and Cross Evolution.)