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I cleared the cache from all cookies and restarted the pc. After a new log in to wordpress: problem solved.

OMG thank you SO much for this!!!
This solved not only my problems of not being able to preview my edits, but ALSO the problem I had that I could not leave comments myself! The page constantly asked me to log in before leaving a reply (which I want users to do, to minimize spam), but even when I was logged in, the comment box would not appear, only the line telling me to please log in first.
I wanted my URL to show my domain name, and not the actual hosting address, so I used the actual URL for the blog location, and my domainname for the page URL. Changed both to http://www.my-domainname.nl, problem solved! Thanks again!!