There should be a in-game setting in mod-menu which can disable multiconnect because it can be advantageous in some situations. For example when a player wants to join a modded server of the current version which would not be possible with multiconnect enabled

So will this be added?

I'm planning a refactor of the way multiconnect handles registries, it's possible it may fix this issue, then I wouldn't have to think about disabling multiconnect.

ok, thanks

You need a different instance of Minecraft anyway in that case.

Please show the error

No, when i am playing on 1.17.1 and enter my server it always gives me an error and does not let me in, but i and my other friends can always enter it with a vanilla client

#216 (comment)
This exact error but with different mods on the client side