Did another check. Enabling Ext-Band was possible if I have reduced the Tx rate from 10 to 8 on the adapter near the TV. Unfortunately the network speed was much lower (avg. 50 mbit/s). So far disabled ext Band with full Tx rate is the best solution and provides avg. 550 mbit/s.

Update 2: changed the band to D-Low AND D-High but without D-Ext and this fixes all the artefacts on TV while the connection remains between both devices. Only the speed is depressed now. So the extended band seems to be the problem here.

Update: I have tested my old TV (Samsung 55" KS7090) on the same wall outlet with the same cable: no issues at all. Tested again with my current TV (Samsung 65" QN90A): again artefacts. So obviously the current TV seems to be more sensitive. Any recommendations to fix that on the current TV?