Ok, after some investigation I was able to find a nasty bug that was introduced in build 21-10 that might have been indirectly causing the issue you were seeing. I uploaded a new version with a bug fix in place (OldSchoolFootballCoach_Win64_2110_1.zip). Feel free to give it a try and see if it fixes your problem! This build should be totally backwards compatible with your saves from 21-10. You can migrate those saves to the new version by copying over the "Data" folder.
The bug I found was that when you select a team and play a game, any football play not in your team's playbook would disappear from the game. This means that plays that should have been available to the other team were no longer there. Closing the game and reloading it would bring the plays back in most cases, but that would only be a temporary work around. But to make matters worse, if you created a custom play in the play designer and saved it, missing plays could disappear for good. Thanks for helping me spot this problem.
I think this might have been the root cause of the issue you were seeing. I'm speculating that you were trying to play against a team with a very different playbook than your team, and so a lot of their plays would become unavailable due to the above bug. The other team's coach might have therefore been trying to call plays that didn't exist anymore and gotten stuck not knowing how to proceed. Also, if you had generated custom teams after plays had already disappeared then the new teams might have ended up with a pretty small playbook to begin with.
Hopefully all of that made sense. Feel free to let me know if this latest build doesn't fix your problem. I'll take another stab at it if needed. Thanks again.