I LOVE MFL!

Already got a response to my email (though they can't help):
"The system will not recognize the 4983 league as it thinks that you are attempting to enter a 5-digit league ID#. It is not recognizing your league name as a number instead of a name. Your owners will instead want to use the commissioner email address to search for the league or you would have to add at least 1 letter to the name to make it be recognized by the system."