[2020] Backend error after season 3 update

Are you playing on the Modded realm? Please switch to the Official realm using the checkboxes next to the ‘Play’ button within the launcher.

If you only get this issue when connecting to another player, then you are accidentally in the Modded realm, instead of the Official realm, like Julia said.

If you get this issue no matter what you do when you open the game, then it’s possible you have a mod enabled that used to be sanctioned, but no longer is. Disable all mods and see if that works. (Or unsubscribe from all mods)