Martin van Dam

Yeah!
Thanks a lot for this link to the upgrade procedure.
It took me about 2 hours but the site is back online and the editor works normally now. It also gave me good insight in how WordPress interacts with the database. I learned something.

Thank you kmessinger, respect.

Martin

After some more analysing I think it has to do with this upgrade. I have WP 3.4.1. installed on another website, so basically the same setup. Here I can switch normally between WYSIWIG and HTML view.
On the site where this is not working I would like to try installing WP from scratch. Anyone any suggestions or advice on what to do and not to do?

Martin,

I also tried disable all plug-ins : NOPE

The “disable visual editor ” box was not checked.
I tried cehcking it and uncheck again later, but still no result.

Martin