OK, not impressed with information presented on this issue so far. :-) I just encountered this problem. Retrospect will not execute so rebuilding catalogs is impossible. (Not a good idea as a first response anyway!)
I discovered that the disk (1 TB) that I send my backup data to has disappeared from the system, i.e., it is inoperative currently - or maybe permanently. Have yet to check it out. Anyway, I went to C:\Program Data\Retrospect and changed "Config75.dat" to "Config75.datxxx" so Retrospect couldn't find it. Then ran Retrospect. It then executed and created a new "Config75.dat" and then was usable.
So, IF I can get my disk back on-line then I will delete "Config75.dat" and then rename "Config75.datxxx" to "Config75.dat" and then run Retrospect and everything should work and be as it was before the disk went off-line.
YOUR problem (whoever encounters this condition) might be different. You might have a corrupted "Config75.dat" file or who knows what else. That is definitely the place to start looking though.
A little more error checking in this area and a little more information presented to the user regarding the ACTUAL cause of Error 641 might be a good idea for a future release of Retrospect?
(I mean, how hard would it be to report the data path name that you are attempting to access and say that it is completely missing.)
Hey, I love Retrospect and have been using it for years! So don't read anything more than "mild frustration" into my comments. :-) Took an hour to figure this out which was 50 minutes longer than it should have taken me... :-)