Confirmed in 1.16.1
The Realm that I have for my family and a couple of my kid's friends is having the same issue. I payed for another 6 months yesterday and about 3-4 hours later, we were all completely blocked out of our Realm with this very problem. We have been playing on this world/Realm for about 3 years now. Something to note with this issue for us (not sure if the O.P. is the same) is that not only does the client crash upon joining the game with the above listed crash report, but also nothing else works either. Meaning I (the owner) cannot change worlds, start a mini game or download any backups, etc..
It's unbelievable after a year a trillion dollar company (microsoft) is unable to solve a multiplayer bug into one of the most successful game.
Tried everything and still cannot play multiplayer.