My nightly backups kept failing with the following error:

Retrospect has encountered a serious error:
Assertion failure at tree.cpp-2528
 
A log of this error has been written to the file "assert_log.utx".
 
Please dell Dantz about this problem.

As with all Retrospect problems, the fix is easy. Simply remove X:\Retrospect Restore Points\RestorePoint.rbc (where ‘X’ is your external drive’s letter) and let Restrospect re-create it. This will often take several hours, so be patient. Once Retrospect has finished re-indexing everything, your backups should resume as normal.

Any time I have had a problem with Retrospect, it can always be fixed by re-building the .rbc file.

I was receiving the following error after completing the setup for Retrospect Express HD with my Maxtor OneTouch External Hard Drive:
Can't save setup, check the selected drive to make sure there is enough free space.
I had plenty of space on the drive, so I knew this couldn’t be accurate. After some research I found out that my config file was corrupt and it was an easy fix: Remove or rename the RestorePoint.rbc file in the Retrospect Restore Points folder on your external drive. Open up Retrospect Express HD and it will re-create a new config file, as well as re-create all your restore points. This can take a very long time (hours) but once it’s done you’ll be back up and running like new.

(Source: http://forums.dantz.com/)