It sounds like you have this logged issue, from the Tech Support forum:
http://www.ootpdevelopments.com/boar...d.php?t=148259
The suggested workarounds that I have read are as follows:
You could click the Action button on the roster screen and choose either 'Ask AI to set up complete Team, Lineups, etc.' or 'Fill Team with fictional players.'
You could try turning on ghost players, in League Setup, which in theory should allow minor league teams to play even if their rosters are incomplete.
A more drastic measure would be to clear rosters for all teams in the AAA league, delete all free agents, and fill the rosters for those teams with newly generated players.
I don't know the status of the logged issue, sorry. But at least you know you are not alone with it.