Are they able to join Vanilla server, It's highly likely could be one of the mod causing the issue, I had my PC server did the same quiet sometimes back and I ended up removing all mod and able to rejoin
Many thanks for your input, I think I've solved it now. The players had to launch the game in Steam and not DZSA and "unload" all mods, then join the server from the Steam launcher and allow Steam to setup mods and DLC for the server. This worked for both players that had the issue and they can now join from either Steam or DZSA.
Something definitely got borked with the mods, the client seemed to think certain ones were loaded when in fact they wern't.