Problem is solved by putting back a complete image of the server. This is some kind of a brute force solution because it was becoming urgent. Consequently I still cannot tell what the underlying problem was..
I will come back if it manifests itself again.
In the meantime I have a problem with another website, where the page remains blank. I'll check first for a solution myself. If necessary I'll start a new thread for it.
Thank you all for your efforts, though.