Hello Werner,
I think I was able to solve it. At least it is currently running. :-)
The ports were/are forwarded correctly, it was apparently the DynDNS.
There it shows me the address of Swisscom e.g. xyz.internet-box.ch and then I had set up one with my personal domain via Infomaniak. When I deactivated this (“Other DynDNS providers”). It worked via the Swisscom address. When I set up the new one from Infomaniak, it also worked and now it works again.
At the moment it works with both.
Apparently the service simply needed a “restart” for the other provider to work properly again.
Before I reactivated the Infomaniak service on the IB 3, I set it up on my Synology for testing purposes and it worked.
Which should I prefer, the DynDNS on the IB or behind it on another device?