I’m having the same issue with OptimizePress (theme and plugin). Every one of my OP sites is unable to be edited.
I’ve decided to do the simplest thing: re-install WP version 4.4.2. Thankfully, I have a copy downloaded to my hard drive already.
Waiting for OP developer to update their software to be compatible with WP version 4.5 is likely to be a much longer — and much more expensive — option.