OK, for those who may have the same problem in the future, here is the solution. It seems to be the special combination of hard drive enclosure/macOS10.12+/Samsung 8TB QVO. I have tried several USB-C enclosures with the Samsung QVO 8TB in combination with macOS Monterey and Ventura, all with the same result: the data system was broken after I ejected the disks, sometimes already while I played a video file. Most of the time formatting was only possible outside macOS.
I then read through
this,
this and
this thread and finally ordered
this enclosure. Now the two Samsung QVO 8TB disks work under Monterey/Ventura as they did with the other enclosures under Big Sur. And why a combination of all three things? As soon as I put a non QVO disk from Samsung into the old enclosure, it works normally under Monterey/Ventura. But since the problem already exists since Monterey, and Apple was apparently already pointed out several times, I see rather black that this bug will be fixed in macOS.
Anyway, now the disks are running again and I can update to the current operating system. Because of this bug in Monterey/Ventura I have been working with Big Sur until today.