Can’t edit Whitelisted User Roles

So, I’ve just uploaded my site on a live server. Since it’s still under construction I used your plugin. The thing is that I want to grant access to an Editor User also. Whenever I check Editor in the Whitelisted User Roles checkbox ant Save changes I get the error:
“The “example.com” page isn’t working

“example.com” is currently unable to handle this request.
HTTP ERROR 500″

on my browser and I can’t save changes. Any idea for what could be wrong?
Thank you.