Thanks Da Flex,
I think the problem is solved. I couldn't get Samba to work, but your input gave me entry points to research. After a fruitless attempt to mount a network share, I checked the NAS settings. The NAS is a few years old, so probably the new Samba version is not compatible. However, in the Network Services on the NAS I found a disabled NFS Service. The rest was easy. After I enabled NFS Service, the Pi found the NAS and the shared folders, but still no content. The next step was to enable the access rights for the shared folders - and SUCCESS
Thanks again for your input