I have good news!
When thinking about the situation, I tried to eliminate and exchange.
While doing so, I experienced that an ol NAS server could not be mounted anymore. and a third one could be mounted. With this third NAS it was sudenly possible make recordings! So it was the NAS connection!
So dispite all the misleading hints and ideas about tuners, it was all about the acces to exeternal storage and not about the acces to tuners!
Now I have the NAS of my choice mounted using a proper set of credentials, and all works fine.
Think you for your input and help, it kept me going.
Balvie