Sorry to hear that, calaverasvisitorsbureau.
It’s difficult to assess from here without seeing the problem for myself, but although you’ve completed a number of troubleshooting steps (which is awesome, thanks for doing that!) I wonder if you could again attempt the following – paying particular attention to the step concerning your theme:
* Deactivate all other plugins
* Switch to a default, unmodified theme such as Twenty Twelve
* Test and see if the problem is resolved at this point
Of course, I’m basically wondering if there might be a conflict stemming from your theme.