Magically, the problem solved itself:
it was enough to open the three ports used by Syncthing in the firewall of the Synology NAS: 22000, 21027, 8384 !
Up until now it hasn’t been necessary as Syncthyng worked flawlessly without opening those three ports …
If anything, now I wonder how it ever worked with the doors closed and if there is a real need to keep them open. . .
cheers . . .
