This is a relatively a rare bug of the game Urban Terror, which a re-installation (After usin CCleaner) will not change anything.
The solution remains a mystery, because it does not work in all cases, but after a few tries,it seems that UrbanTerror isn't searching for the list in the right place.
For some reason, the master server no longer provides the lists. A simple solution can be applied by choosing an auxiliary list and access the lists:
In the selection screen for the servers, tap the "²"
The console will open, then type /cl_master master2.urbanterror.net/cl_master master2.urbanterror.net
Press the Enter key.
Urban Terror will then attempt to access the Auxillary Master Server (masterserver2), to get a new list.
If this does not work, try this:
In the console, type /cl_master master.quake3arena.com.
It is the old server of the Quake 3 Arena game, still in service.
Note that after a few days, it is possible that UrbanTerror have the same problem again as the auxillary path that you have opened will be closed. Then type in console /cl_master master.urbanterror.net to locate the master server that has been temporarily stopped.