Thanks for your answer.
I have re-checked some of the pages which gave errors right now, and (miracle?), I could not reproduce it again.
This is good news, but I don't know why I could find no error after trying about 25 pages, while I got more than 1 on 10 pages yesterday.
I tried some modifications, and I did systematically check the pages after each modification in the code (based on the information I could find on the web), and I systematically got the error (again, it was randomly, let's say 1 page in the 10).
And I regularly cleared the whole cache, to be sure the error did not come fro cached content.
So I stopped looking after the problem, waiting for your reaction.
The only thing I did, was creating a few new banners, assign them to a specific kind of pages, and disable the existing examples.
Not sure this helped, but anyway I cannot reproduce the error now.
I'll keep you informed if ever that error happens again.
I noticed something else, may be this has nothing to do with the problem, but who knows....
The site was (in my opinion) abnormally slow the few last days. I have now the impression (but this is subjective) it works a little better today. Could a problem on my porvider's side cause this type or errors? Some time out while fetching the needed information...? No idea.
I have created a super user account for you.