So let me start with this: I have been running 3.5.1 for a while and what I will describe now used to work. I have since not installed a new plug-in and those 3 plug-ins that received updates since my last post (when it was all working), are not at fault (I disabled them to see what happens; no effect). I have also not made any changes to the code otherwise.
When creating or editing posts and using the visual editor I am now unable to
1) add any tags to my post (if I click on my previous tags nothing happens. I used to get a list of previously used tags but not anymore)
2) If I click on "add media" nothing happens. I used to get a lightbox where I can upload or select media to be included. Not anymore. I am unable to add any image other than the "featured" image
3) when I try to add a hyperlink in the visual editor I get the lightbox with the hyperlink details. I enter my link and alternative and once I click OK, the box disappears and I am redirected to the "show all posts" overview. No changes have been made, no link has been added. When I manually add the hyperlink in text mode that works just fine.
I disabled the popup blocker in Firefox. I also tried in IExplorer. All to no effect. Any suggestions?