Sorry about the lack of information..I should know better.
I'm running Retrospect 6.1.126 on a new Mac Pro running MacOS 10.4.9. (I had the same issue on my Sawtooth G4)
This happens with Linux and Windows clients. I'm running client 7.0.110 on Linux and 7.0.112 on windows.
I get the same elem16.c-687 assertion failure.
I do not have two copies of Retrospect running.
I'm backing up to File on an external Seagate Firewire drive.
I have a client that is in this state now. I can reproduce easily by going to Configure->Clients, click "Configure..." for this client, then click on the Configure tab in the "Client Configuration" window. As soon as I click the Configure tab I get this assertion.
If I remove the client and add it back, it will work for awhile, but will eventually get back to the same state. (it would not be that big of a deal if I did not have to go back and re-create all of my sub volumes)
Let me know if I can provide any more info. I'll keep this client in this state for now...