It's been some times that I've noticed a problem when the sip-communicator.xml file is written at exit. Sometimes a big part of the file is missing, thus at next startup, sip-communicator fails to read it and ask again for account registration.
I experienced the problem again today on both MacOSX and Debian. Is this a known problem here? I could not see any issues in the tracker, I may have missed it. I can report one if needed.