The most likely cause of an error on uploading data with regard to players happens when someone deletes a player on the web side after you started your game (or since your last sync to the game prior to starting the game). While this should not happen, people make mistakes and we have seen the issue in past seasons.
If you get an error dialog as follows:
The stats will continue to upload but will not contain the information for the player noted in the dialog. The # number is SportsEngine's number for that player and while there very will could be a player with the same name and jersey, the number that distinguishes the player is not there. Repairing this on the device and re-uploading the stats is an option when time permits but it will require working with us directly as there are several steps involved. Contact us as firstname.lastname@example.org to resolve an issue like this.