It is very disappointing that this issue has yet to be fixed. I want to add that the comment by PointedPhoenix8 (method 1) is probably the only way you can "fix" this issue. If you can copy the your world to your local computer and toggle the Update 1.21 on, then copy back to your sever location this will fix the issue. The only real drawback is once the experiments are turned on you cannot turn them off.
If you have not turned cheats on your world, the experiments on will still give achievements if you are concerned with that.
I did all the updates to get the same issue, including the 1.20.80, but with the experiments turned on in your world file, the server should not have the issue moving forward. I have been able to play for about 2 weeks now.
I hope this helps, it does not appear Mojang/Microsoft will fix this issue before 1.21 comes out. the only hope is that having experiments turned on does break something when 1.21 comes out.
Having same issue now, the game would crash on me exiting realms prior to 1.14.60 hotfix, thought it was a realm issue, tried to play in a local world, crashed as soon as I went to sleep.
I have this exact same problem as soon as I load any world single player or realms it immediately crashes and freezes