Friend can't join

Currently, me and my friend have been through a 2 hour hazzle, because whenever he joins any aternos server, he gets the error message: internal exception io.netty.handler.codec.decoderexception.
Since he recently migrated, i got him to change his xbox settings to allow him to join servers. This was not effective.
He has been on one of the servers for roughly 1 day (of playtime), yet for some reason, he can no longer join them. This new server i've made, was supposed to be our modded playthrough, but before anyone comments on it being modded, go back to the start and read-

Anyways, i desperately need help here. I don't have his crashlog. If you need it, please tell me, i'll get him to inevitebly crash once again.

Sorry for the late answer, don't go to aternos support that much. They are able to join with the DYN IP. He migrated when he had to, when mojang accounts where no longer supported. He is on pc as am I. The Software is, as said, different. 1.19 and some modded. And with modded, i mean i made my own pack by downloading a shiz ton of mods, getting the cores and getting through all the errors, so i was left with a handy little pack that we were going to play. Which we ended up doing.

As of the screenshot, i am currently unable to send anything, as i'm away from my computer, but since he can join, i don't think i'll need it. Although i will say thank you for trying to help :).