From page 153 in the Retrospect Users manual, -108 is an "Out of application memory" error. This can be fixed, but you need to know more about the environment of Retrospect first.
Retrospect asks for 4,000 KB (4 MB) of finder application memory by default. This allows Retrospect to back up about 10,000 files and folders. We suggest setting this finder application memory request value to 8,000 KB (8 MB). This allows for the backup of 32,000 files and folders which is close to the upper limit for Retrospect's back up capability.
Make sure that you have configured your "Memory After Logout" value, in the Virtual Memory section of the Memory control panel, to match the memory tuning required for the amount of physical memory that is installed in your server. If you have only 16 MB of RAM physically in your server and you have your AppleShare Pro cache set to consume a large amount of RAM, then you will get a -108 error from Retrospect.
Adding memory to the AWS 95 platform is the key to improved performance. We have found that the absolute minimum memory that customers have installed, and are functional is 24 MB of RAM. We suggest somewhere between 36 MB and 48 MB would be a more reasonable amount of RAM. Anything over 48 MB (depending on tuning and intended function of the server) should be tuned to be allocated to the A/UX I/O Cache and will improve performance of the server noticeably.