I just built this modpack and I'm not 100% sure that the issue isn't a compatibility one, but thought I would share it here in case others have the same issue.

I claimed my horse, got off and swam across a river, then tried to call it and got the message that I couldn't recall it while riding. Thought maybe it was a weird glitch so exited out of the world and tried to go back in, that crashed my whole game and that world is now corrupted and will not load at all.

I'm sure logs would be helpful but I'm not 100% sure how to get them and had to force stop my game so not sure if there is a crash log or not. I'm on 1.16.3 with forge 34.1.23 running on Linux. I'll try and update if it happens again and I'll see if I can figure out the logs thing.