Sometimes this error occurs when you have conversations enabled. If you do, turn them off and see it that solves the problem. If it does you can turn conversations on again after deleting messages and it should work okay.
There is no limit on the number of pro leagues a manager can join. Joining what would be considered a lot of leagues by some is not a violation of any Yahoo terms.
"no person can watch 120 teams and make several moves each day without something fishy going on"
You state this as if it were a fact. It is not. Some people are willing to invest the time to manage their fantasy teams. This is also not a violation of any Yahoo terms.
If you truly believe that a manager is breaking any Yahoo rules, contact them at this link:
You as the accuser should provide some legitimate evidence as to why you are making your claim.