I have similar problem.
Try using browser developer functions to see what console errors you get - if it is same as mine you get a problem with /themes/style.php?stuff.
Try opening the failing URL in a separate tab and see what you get.
Depending on the theme I try this with I get either a 500 error or a Stack Trace.
(My help request is at https://moodle.org/mod/forum/discuss.php?d=356071 )