SOLVED
I have exactly the same issue here, only with me it occured after migrating to another URL....
If I turn off debug mode, a 500 error appears. If i activate debug mode, the backoffice works, only with a messed up menu, if i push Ctrl+Shift+J in Chrome, I see a lot of Jquery issues. I checked the version in defines.inc.php and that looks good as well......
I hope someone knows how to solve this.
SOLUTION:
I redownloaded and re-uploaded everything (files & database), now it works!