I'm experiencing a strange behavior with a site after upgrading from 8.3.0RC2 to 8.3.0: i'm able to successfully login as admin, i can add blocks, i can go to System and Preferences, i can call any system page, but when i try to enter edit mode for a page, i'm getting a blank page due to a 500 error. This is happening with all browsers i have been testing. Chrome for instance says: "This page doesn't work. HTTP ERROR 500. Also, the issue persists with the default Elemental theme.

The only way to enter edit mode is to

1) click on edit page, which results in a blank page (500 error)
2) click browser's history back button
3) click browser's reload button

now the page is in edit mode.

Has anybody had a similar problem or an idea what could cause that issue?

Thank you,
Michael

---

UPDATE 2017-12-22:

I was able to solve the problem by changing some small things in the header of the theme. Unfortunately i can't tell anymore which correction actually did the trick.

At least i can give the information that the 500 error had been caused by a bug (maybe typo) the active theme's header.