Hello,
Disclaimer: I am not an expert on anything.
To my understanding, UTDC used a gamespy master server to generate the server list.
If the server list via DNS tricks or similar can be obtained and the maps are not loading, it's probably related the server version *or* the map mismatch.
Some maps are indeed different but I am not sure if the changes are in the maps themselves or just how DC version rendered some things visually. The teleporters do look different on all maps for Dreamcast. However, this might not have anything to do with the individual map files. Where as other differences between the maps may be changes in the map files themselves.
For few examples:
CTF-Niven PC version at both bases, there are two blades spinning outside of the enemy base door. On the DC version, this is just a wall but you can still use your translocator to hide inside these walls, only on the DC version.
CTF-Face DC version red base let's you walk behind a certain wall on the main floor.
To confirm where the changes are, the map files would need to be ripped from the Dreamcast ISO and opened in UnrealED.exe comparing to the official PC versions (such as CTF-Face, Niven, etc)
The DC version did include a few addon/bonus maps like CTF-Face[] (Face2) but there are also a couple of exclusive maps I think I have only seen on the dreamcast version. (My memory isn't 100%)
On a side note, the music is also slightly different. While the maps run their music off CDDA tracks, some maps music themes are shorter compressed versions than the PC versions as can be heard by playing the UT DC disc in any Audio CD player.
The DC differences only apply on the Dreamcast version. The PlayStation 2 version mimics the PC version.
I hope this helps anyone because it would be a great treat one day to play the old UT online maps on Dreamcast with the Dreamcast keyboard and all