I am running
Wordpress version 4.9.8
NextGEN Gallery 3.0.6
NextGEN Gallery PRO 2.3.5
When I try to save a newsletter I get the error:
"You need a higher level of permission. Sorry, your are not allowed to customize this site."
I also checked the chrome debug tools -> Network tab and I see that its a "500 internal server error". When I deactivate NextGEN Gallery PRO I can save the newsletter fine.