So, I simmed ahead about a decade and created a new quickstart file from that save. During the next season, I get
this prompt while trying to advance to July 2.
I'm pretty sure those leagues can't be edited, so I appear to be stuck.
I haven't tested to see if my non-quickstart save runs into the same problem. (I'll probably do that after I post this.)
Bonus bug:
I think some custom jerseys are breaking player cards. I'm seeing this in the sim and game screens.