I had this problem for a while. What I did was go into the preferences menu, and look at the User Control tab, then switch all of those over to Administrator. It worked for me, hopefully it’ll help you.
Strangely, my site was having the same issue despite being set to PHP 7.0. It was so bad I couldn’t even get /wp-admin access without getting a 500 error. Just on a lark, I went into my hosting settings and rolled it back to PHP 5.6, which fixed the issue. I’ve now deactivated Jetpack and rolled it back to using 7.0. Hopefully you get to the bottom of this soon.
