https://paste.dimdev.org/moxesutopu

Both the client and the realm are 1.15.2

here you go https://paste.dimdev.org/odirimimuk

Please test with multiconnect on its own and let me know if the same error happens.

Ok so I did that and I was able to connect, I also tested which mods might be causing the problem and I have narrowed it down to Immersive Portals mod. Here is the jar for testing (remove .zip extension)
immersive-portals-0.12formc1.15withfabric.jar.zip

Does it happen even without realms?

Can you send the whole log?

Verified it may be with Immersive Portals. With earlier tracking, Immersive Portals is modifying the PlayerPositionLookS2CPacket, and throwing in Multiconnect from it (Best guess from looking at this)

if i connect to a normal server? I get this error when tring to connect to us.mineplex.com https://paste.dimdev.org/wudajejeqe.nginx

and this error when connecting to play.dawnhaven.net https://paste.dimdev.org/oniquriliz.css

https://github.com/qouteall/ImmersivePortalsMod#breaking-hacks
Multiconnect is not compatible with both-sided mods in general, let alone those with "breaking hacks". Please use a separate profiles for multiconnect and immersive portals.

Understandable, I will ditch Immersive portals since I dont really need it anyway, thanks for help!