I have the same problem too with my project league, overhaul my country league. I already try to untick the number of dates, and set the start month one month earlier, and end month one month later, and still get the same error. I even don't know where these number is coming from
Waiting advice from here first, before attempting to split the league one tier more to make the match dates of the upper league lesser too
Indonesian League Level 5 Fantasy (Verified).fmf