The problem has been solved for me, which I am very, very happy about.
But it has nothing to do with the PM+ version, I'm still working with the latest version.
During many other tests, I noticed that the problem with the thumbnails started whenever I was looking for pictures from a specific folder. The problem remained even if I was looking for pictures from another folder. Only after a restart of PM+ everything went fast again. In this folder were 301 Nikon NEF files and 301 associated XMP sidecar files. Accessed via the Navigator the images of this folder had also much longer when loading. Lookend at this folder in Windows Explorer I couldn't see anything unusual (no other files in the folder and also the file sizes were inconspicuous).
How did I solve the problem? I simply changed the name of the folder and then shut down/restarted my Synology NAS. After that I gave the folder the old name again. From that moment on, I had no problems. I'm not a professional when it comes to PC, but maybe there was just some kind of address problem that was solved by renaming. Probably it's more likely the problem was the Synology NAS and a reboot was necessary. In any case in the future, I will restart PC
and NAS when new problems arise. I would recommend that to you, Andreas, too.