Gotcha !
No unwanted interrupting stops anymore !
Although I do not dare to say “that’s the solution”.
Maybe in a few days…
I continued searching the internet and the pages of AVM/Fritzbox.
There were comments on the new WPA encryption “WPA3”.
My Fritzbox was set to “WPA2 + WPA3”, which means WPA3 capable items use WPA3
and the others use WPA2.
I thought it’s worth a try and set my Fritzbox back to “WPA2 (CCMP)”.
Success, not stops anymore !
As said before, I don’t dare to say “That’s it”.
But maybe my old (late 2013) iMac is not capable to work with WPA3 (although he switched and showed WPA3) and the router.
However, it works.