Due to an occasional fault in UniAdmin, an addon may be listed, but the file has been deleted (see http://www.wowroster.net/Forums/viewtopic/t=5175.html for info)

If UU can't download the file, it raises an exception within the log and doesn't progress. This leads to UU halting mid-sync and doesn't update the interface's buttons to allow further actions.

Seems there are two options:
Carry on downloading and completing the syncronisation
Halting the syncronisation, but re-enabling the interface.

adding to the todo list, although I've been working on a new tool lateley, that may replace UU