Hey there. I'm running a server for me and a couple buddies. The only mod I really run it with is the simple voice chat mod. This meant that people who wanted to just play vanilla could if they wanted, without installing forge, and there would be no errors. Then I updated the server to forge 1.21.5, and things got weird. People who play on vanilla get this error:
I haven't added any new mods, I only upgraded to forge 1.21.5 (switching to vanilla would make the server provider reset the whole world, not really something I wanna mess with right now).
One of my friends managed to get in on some cracked version of vanilla (before he was kicked out of the server) but his whole world and inventory were corrupted, which is weird because when I went to his exact coords the world was fine, and when I /killed him to try to get his stuff, his inventory was completely normal.
Just wanted to let the devs know about this. Hope you guys can fix it in the next version :)