Hi MissTeriLynn,
Thanks a lot for your fast response!
To answer your first questions: The printer is connected via USB directly to my computer. I did try to copy a document and that worked just fine.
I did what you suggested. After rebooting I found out that the printer was there (again) in the 'printer & fax' window, even though I thought I had it removed by resetting my printing system. So I removed it again manually and added it again.
When I launched the 'HP Scan' software it gave me, unfortunately, the same kind of problem as what I started with.
But then I noticed a 'Scan' tab in my 'Print & Fax' window and found a button 'Open Scanner...'
Totally unexpected it opened a new window and started to scan the document that was in my scanner! So I think I did find a way to scan anyway. I tried to open 'HP Scan' again, but to no avail. So there seems to be something wrong with this software itself, because the scanner seems to work fine. And maybe it did work fine all the time without me knowing it.
Thanks again for your reply. The urgency of resolving my problem is a bit lower right now, because of this strange but nice solution I discovered. But I think it might be nice to have the HP software itself work properly as well. So if there are other things I can do to test this, let me know.