I had the same issue. The solution is to delete your Mac's OPDS profile in Marvin and create a new one, according to these instructions:
http://blog.marvinapp.com/post/49081628921
Note that when you enter your Mac's "IP address", that's not your web IP, it's your WIFI IP. Go to "About This Mac", choose "System Report", click "Network", and take the number to the extreme write on the "Wi-fi" row (e.g. 192.168.x.x)