It’s hard to say what could be causing the issue without knowing your configuration, however, after some research, some sources suggest that this error could be related to the DFS channels. Could you try navigating to Network → Wireless, editing the Global 5GHz instance settings, and change the channel to 36 or any other non-DFS? Make sure to set the Country code to the country you are located in yo comply with the local regulations!
If that does not help, please post a screenshot of your 2.4GHz and 5GHz WiFi configurations, as well as the Network → Wireless page.
The 2.4 GHz channel is locked to 5 (cannot be changed, as it is used by the wifi connection to my Fritzbox). The 5 GHz channel is set to Auto and appears to be 149 at the moment. Please see my screenshots below for details.
I don’t know why, but the last message of Wrong coupling... is dated 10 Oct, and I have not changed the channel since then. Does this mean that the currently selected channel 149 is poor but unlikely to have caused that warning?
And is my understanding correct that instead of keeping auto-select, manually setting that channel to 36 and keeping the width at 80 MHz means that the overall bandwidth then spans channels 36 to 48 (see below), i.e. just outside the DFS-regulated channels (in Germany)?
Well, if the issue ever reappears, try the suggestions, as the router might have switched to a different channel afterward and the issue disappeared.
Correct. DFS can be quite complicated, but it’s purpose is to scan for any radar presence, and if it’s present - switch channels. DFS spectrum is quite large, as different radar frequencies can be used in different countries/regions.