I am too having the same problem of getting the error -530 for the past few weeks. The error I'm getting is:
Can't access volume xxx-macHD on xxx iMac, error -530 ( backup client not found)
1/1/2010 1:02:07 PM: Execution incomplete
I tried restarting my Airport Extreme, but unfortunately this didn't fix the problem.
I am running mac os x 10.6.2 on both computers. The retrospect version is 8.1 build 626 and the client is 6.3.027.
I can access the client computer from the finder without any problem. In retrospect, however, I can not browse the other computer from the "sources" screen.
Any suggestions? It seems odd (and frustrating) that I can use the finder to get to the other computer, have the client running on the other computer and be getting this error.
Thanks in advance.