I had problem with offline update.
always information:
The operation failed.Please log in in to DSM again and retry.
So. In this case that wasn't problem with firewall , or connection between NAS and PC. Problematic was browser with plug-ins, or maybe... Opera.
I try with clean IE, and Firefox-- update successful on both.
i had problem only on opera 12.17 and 12.50, maybe with opera 25 everything will be ok,because this is not real opera
BR