Hi Roger,
It's not that linux clients can't connect, it's that modified clients cannot connect.
If you get the symptoms you describe it means that you are not using retail copies of .u files in your setup. i.e. they have been changed in some way and are not the ones published by Epic. You're trying to run modified versions of the files.
To fix this, and at your own risk, you could try the following:
1. Backup your ut2004 install. If anything goes wrong you need to be able to revert to this
2. Replace the .u files in your install's system directory with retail v3369 versions. I have uploaded these here:
https://miasma.rocks/downloads/3369_u_files.zipOnce your copy of the game is running with original v3369 .u files you should be able to connect with no issues.