Exception Cannot create file ""
Means, I presume, that the file being decoded has no recognisable filename, or illegal characters in the path, and/or probably points to a bad post. Never seen this before, so not sure.
Somehow I don't think you'll be seeing this one often. If you do, we need to press the Rdl Activation Button.
P.S. Remember that NZBs are just information files: lists in xml format of article parts and how to find them on your server. Once added to the queue, you don't need them anymore. Certainly there's no reason for your default download folder to be the same as your NZB folder. Actually, download, NZB, autoimport, temp and unrar/destination folders should all be set to be different.