Why I can see the thumbnail-preview from PSD-files, but when opening in viewer format is unknown? AFAIK thumbnails are saved in psd, but is viewing version related and which versions are supported (I use CS5.1)?
It depends how the psd was saved.
Thumbnails aren't the same as preview images in psd files, they can have both, or either or neither.
When saving the psd, make sure "maximize compatibility" is on.
For some reason I deactivated "max. comp." some years ago, this explains, why some files can be viewed and some not.