' update post' does not update

When i edit an old post, and press UPDATE, it says post updated at the top. However, the text is still the old text!

what i tried so far:
– disabled almost all plugins except the standard ones.
– adding and removing a space at the title (suggested in another thread)

the only solution so far is to actually remove the post, and paste a copy to edit before saving, all the while making sure the title and slug are the same to keep my backlinks working.

Suggestions are welcome, as this is very annoying!