Di dorearendil (and every one else stuck with this problem)!
I found very simple solution: switch to default theme and deactivate all plugins – it will surely work! then try your theme. if it works, start activating and checking plugins one by one.
In my case, I chose “WP UI – tabs, accordions and more” and in the end it works fine with all the other plugins. it seams that the order of activation is what matters.
good luck, and please report me the results!
best regards,
Ivo