What component of SEP are you disabling so that you can access the NAS again?
If you can narrow this issue down to one particular component (e.g., the firewall), then we can provide better recommendations on how to remedy the issue.
The simplest way to do this would be to open the SEP client, and select Options next to one of the different protection technologies, and then select "Disable". Do this one at a time until you are able to access the NAS.
Let us know which technology you have to diable to access the NAS.
James