e-Newsletter error: Cheatin’ uh? Sorry, you are not allowed to customize this site.

Hey Team

When I try to create or edit a newsletter, I'm getting the error "Cheatin’ uh?
Sorry, you are not allowed to customize this site."

After this error, a new newsletter will have been created but it is uneditable. It will send too, but it's not much of a newsletter...

I've searched the forums and although other's have had the same error I haven't yet found a solution.

Steps taken:

1. I've updated my theme (from Elegant Themes) error was still there.

2. I have deactivated all plugins (5 at a time) to see if another plugin was causing conflict. The error was still there after refreshing the page each time to check again, on all plugins.

However, although the "Cheatin' uh..." message was still there, when the Simple Cache plugin was deactivated the error changed to "The requested theme does not exist". This did not go until Simple Cache was re-activated and then turned back on.

Another odd thing, when I deactivated, tested the e-Newsletter and re-activated Membership 2 Pro, a message appeared saying "Membership 2 Pro is set up for version!" I checked and the latest version is Could this be an issue?

Any ideas?

I have enabled support access in case anyone would like to take a closer look.

Thank you!