Ok... problem solved...
No, please don't ask how...
ok then...
I recently changed the network in our house (2nd WLAN Router for smart home devices). Of course the IPs have been changed on this occasion as well. So in fact, the Synology provided all the content to our tumble dryer instead of the Kodi-Pi... Seems I now have a legit justification when my wife asks me to do the laundry and its taking a bit longer (due to the goddamn tumble dryer watching netflix episodes instead of drying the laundry!!!)
The reason why I din't get to this point earlier was, that the NFS content was visible in Kodi and I thought it would be only visible to the defined IPs, but instead it is visible to everyone but with restricted access.
Shame on me Thanks Klojum for your time and advice
(btw: I reset the synology export file so it looks the same as before (with "insecure" and "insecure_locks" at the same time - working!