deactivated ALL plugins and reactivated one by one but this did not resolve the issue.
update to 3.9.1 did not change anything.
as for your 2. suggestion: if i do this and then revert back to my custom theme, do i have to customize (like widgets, links, etc) everything again?
suggestions 3 – resetting plugin folder… all my plugins are activated. there are no inactive plugins anyways.
last suggestion – no idea how to do it.