I don't believe it! A week of full shutdowns/startup's and even a reformat with resending of all books didn't solve the issue, the series information was and remained gone on the Sony. However, after I checked all the options in Calibre, as per your suggestions, without changing anything, as everything was OK and on automatic, I added an additional three epubs and restarted to get back to PRS+ and lo and behold: Everything was back to normal
I have no idea what changed or how the solution came to be, possibly because of the positive vibrations of all those wanting to help, I don't know. But the important thing is the reader up and running again as it should!
Thank you very much all of you for your suggestions. I learned a lot because of that!