I took some days before reporting again... Since then I installed:
25.07.1-RELEASE (amd64)
built on Fri Aug 15 20:42:00 CEST 2025,
and the issue re-appeared but did not (yet) clear by itself.
Wireguard works well, that system has only one tunnel with another site, using interface assigned mode. I depend on it, and it works flawlessly.
Yet the wireguard configuration page, its status page as well as the services widget on the dashboard, all report the Wireguard service as stopped, with the usual icon to start it. On the dashboard the gateways widget shows Pending for the IPv4 and IPv6 wireguard gateways.
On the other hand, and it is logical as the packets do flow through the tunnel, the Wireguard widget on the dashboard shows the tunnel up with traffic.
To be complete, the watchdog service, if wireguard is configured there, spend its life at detecting wireguard stopped and attempting to start it, which does not work.
The other side of the tunnel is still on 25.07-RELEASE (amd64). I'm reluctant to upgrade that side too, with fear to loose the wireguard tunnel altogether.
Aren't there any logs from that wireguard package? I can't find anything of that kind.