1. First and foremost, make sure that you and your league's commissioner are using the exact same version of OOTP. This can be found in the upper left corner of the screen right after loading up the game. If you do not have the same version, you will likely be unable to open the league.

2. If you are trying to open up a league for the first time, make sure that all of your folders and files are in the right place. Your saved game folder should end in a .lg extension and should be found directly in your data\saved_games folder. Inside your .lg folder should at very least be 13-17 files ending in a .dat extension. If your .lg folder is not in your saved_games folder and does not include these .dat files, it will not open.

3. If you are still having trouble, ask your commissioner to send you an entire fresh set of league files and try loading that.

4. If you cannot download updated league files in-game, make sure that your commissioner has finished uploading them first. Also, you can try disabling your firewall or anti-virus programs momentarily to see if they are causing a conflict.