Yesterday night I 'had to' restart Windows after the monthly batch of updates.
The fourth WU I mentioned in my last post had been running for more than 2 days, while the real CPU time remained at 3h03m all the time.
After the restart this WU restarted at a point close to this 3h03m, and both elapsed time and real CPU time started increasing.
Meanwhile this and 2 of the WUs I mentioned in my first post have successfully completed.
So, conclusion is, that the restart solved the problem. But the mystery remains: what caused these WUs to act like this?
As this turns out to be a local issue, we will probably never know.