I used to work with the UA-25 ex on Linux and it was flawless in jack and alsa.
<3 linux audio
So cool, as there is already lots of good stuff coming with it.
For Jack I worked with QJackcontrol, here you can also do the routing (or with Catarina, Catia or Claudia?).
Make sure, the buffer ok and latency settings are big enough.
It seems that interface has two different modes of operation, basic (USB
audio class 1.0) and advanced (seems to be proprietary, not fully USB
audio 2.0 class compliant), and has hardware controls to set sample rate.
no the contrary, as I remember, advanced mode has to be turned off before connecting or booting.
You can also try Renoise, Bitwig or Pure Data to know if it's a problem of Ardour
(and maybe start them from the terminal for reading errors and you can look into their logfiles, too).