The "rotamer library" is named rotlib.bz2. This file does exist in your DF directory, correct?
How are you running the client -- as a service? If not, did you shut it down before rebooting? How'd you shut it down?
Which OS?
If you have something that can read bzip2 files (note: winzip can't, last I knew, but almost every Linux machine in existence can, with bunzip2), try decompressing the rotlib.bz2 file (then compress it again), to make sure it didn't get corrupted.