Hi
@Toupz
Nice to see people trying to support Enhanced Client. How is it appreciated on your shard ? Is it something like 20% EC and 80% Classic ? Any bugs to report on Publish 58 ?
I thought I'd try to help you here, as I'm trying to get old friends to UO and I'd like them to play with EC.

Also, I'm glad to help when I can.
First: which ECLauncher you trying ? The one from Argalep ? Is it the one you can type the IP and PORT and let's you get in ?
There is another older version where you can modify a CFG file (cheese.cfg), type the IP in number format, and seems to still be working currently. Although, players new to UO seem to prefer the one where you don't have to edit a file. I think you can still try to find the cheese.cfg file somewhere on this forum (or JustUO's archives). This was a post from Zerodowned if I'm not wrong.
I'm sure a Linux guy can edit a cfg file ... but the big question is why isn't it working on Linux ?
So my second: what is he using to start UO ? Wine, MONO, a virtual machine running Windows 10 ?
I don't know linux/lutrix myself, but I'm a frequent user of Debian. Linux can be a pain when it comes to inject an assembly in an active process (especially Windows processes).
That's why I tried to code the launcher from an older version, whose code was published on an abandoned GitHub.
Here's an anonymized version attached to this post. It may trigger virus/reputation alerts if you use a firewall, but all I did was using EasyHook to inject the IP to EC client. I can share the code if you want to check

and if you have ideas to improve it. I'll check more into EasyHook and try to find something Linux friendly. Hopefully an updated MONO version fixes your friend's issue though ^.^
(btw, this launched I made doesn't require an installation. Let's say it's portable. This fact may alone fix your linux friend's issue. hopefully !)
Good luck o/