For a short time I have had the problem that no more apps / updates can be loaded in the Google Playstore and also Youtube no longer plays videos. All other things that I use on the net work so far without problem.

A search on the net has shown that it is probably related to the load balancing of Google.

see also  Google -> Reddit play_store_constantly_pending_and_youtube_videos

If I connect my devices via mobile Internet or other WLAN access points, everything works without any problems, just like before with my device.

At the moment I'm at a loss, in the user interface of the device I can't find a point for filtering exactly this URL, via mobile internet and other WLAN devices the call works and one gets the expected "404" error,  as described in the Reddit entry.

What else can be done?

one more additions:

• the router runs in double NAT, you connect to the WLAN of the ISP router, then there are no problems, so I exclude a problem with the ISP
• the DNS settings have already been synchronized between the ISP and the router and no differences have been detected
• a ping to the gvt1 domain resolves to an IP
• everything worked wonderfully in the current configuration until 6 days ago - no firmware updates of the router etc.