Right, using new league name, tried using a same name once, and generates a different error saying league name already exists.
So tried importing 2001 with the lahmans4.5 database using OOTP5, get same error.
So to make sure my system could handle these databases, opened OOTP4 and imported a new year, and worked fine. Pretty much stumped now.
In summary my OOTP5 can not import either lahman45 or lahman 50 data, but my OOTP4 can. Thinking I'll just go ahead and start with fictional players and not spend too long trying to import seasons for now, anxious to get simming.