How do you upgrade?
I hope with unzip.php.
No, unzipped on UNIX on the server side as usual.
But it seems that this was a cache delay rather than code problem. I renamed the "templates" directory to something trivial ("templatesEVP"), reloaded the backend, then renamed the "templates" directory back as it is
Thereafter I reloaded the Backend again, and now it seems to work right nicely.
That procedure solved the problem. Thank you for your time an sorry for the inconvenience!