I got the answer to this through the dutch forum section.
Just for future reference if somebody finds this one with the same problem.
I had to change a few permissions to get the backend editor to work properly and also to get w3 total cache to work. I was a bit lazy so instead of selecting the right files I changed the permissions to 777 for my whole public_html folder. Although I changed it afterwards knowing this was not a great idea security-wise.
But in doing this I did it recursive and chose to do this for files and folders. This made everyfile also 755 while it should be 644 for files and 755 for folders.
I changed this with filezilla (recursive and now choosing only files and only folders for the different permissions) and al my problems were gone.
