Inconsistent errors when connecting to MP

[Ser_Birdman]

Hi y'all,

I run a headless server in unraid and last month me and my buds finished a vanilla run then decided to try our hands at a Bob's-Angel's modded run. At first everything was great, and then at the 1.21 update two of my four players could no longer connect. They get a black screen with "trying to migrate server settings..." then the connection fails. Comparing the server logs to their local logs there is a very long sequence of invalid packet warnings. They have tried:
1. completely fresh installing from steam
2. deleting all files including appdata files and then reinstalling
3. deleting all files, uninstalling on steam and then installing from factorio.com
4. reverting to vanilla server to make sure it wasnt mods
All leading to the same outcome, they still can't connect but my two other players have no problems. I've tried tweaking server settings, disabled user verification, removing the password and the game looks like it is TRYING to connect them but something fails. See this from my POV already connected https://i.imgur.com/3L5jmqN.jpg

Literally no idea what is happening here. I'll attach snippets of logs from both serverside and clientside.
Serverside: https://pastebin.com/WmrprsrU
Clientside: https://pastebin.com/7fCrdNS4

As I said, server was working fine in Vanilla a few weeks ago but now half of them can't connect. Port forwarding is correctly set and my router logs seem fine. Is this a firewall issue on their side? Any help you guys can give would be amazing.

Thanks!