I might be having a similar problem as you. I am using a ReadyNAS NV+ with 4GB with Retrospect 7.6.111 and Windows Server 2003 Std SP2 fully patched. Are you getting a "Sorry, server login failed, error -1116 (can't access network volume)" and "error -102 (trouble communicating)".
Does your READYNAS unit require a manual reboot for access after Retrospect locks up?
If so, are you sometimes stuck in a seemingly endless loop of Retrospect dialog boxes that do not let you stop the executing script until the ReadyNAS is rebooted?