Thanks Coro and Bucking_Horn for the explanation. I just think as long as the problems are fixed as soon as I take Pi-Hole out of the equation im pretty sure its my setup with pi-hole and not the Upstream DNS. I had to take Pi-Hole out as the broadcasted DNS-Server on my LAN because my Fam started complaining about the outages. Im back at testing with my devices only and as manually set DNS-Server. So far without any problems. But the fact, that the problems stopped imediately after these change I am wondering if something is wrong with the broadcasting of Pi-Hole as DNS-Server. I have some variing theories about that:
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I am using a second fritz.box router as a W-Lan extension in what the manufacturer calls "Wlan-Mesh". I realised that at least yesterday I had some issues (SERVFAIL for DuckDuckgo.com and others) as soon as I was connected to that second Router. Now this router did not have IPv6 enabled which I did now. But I am not sure this got anything to do with it at all. Maybe it simply does not broadcast the DNS-Server properly? On the ohter hand this could really not have been the case, when I had the MS Teams issues, because I was in the basement and the second router is all the way on the second floor. So I was defenitly connected to my primary router.
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DNSSEC. I keep going back and forth with this setting in the Pi-Hole mainly because I sometimes have connection issues with a Dynamic-DNS address that I use to reach my selfhosted Nextcloud. Mostly when these issues emerge and I switch DNSSEC On I can reach Nextcloud again. I then keep switching it of, when I have other issues. In general I am not sure if Pi-Hole should be handling DNS-SEC or if my Clients do that anyways. (I also wrote about it here in german)
In general right now there is just to many moving parts and its hard for me to figure it out because some issues only emerge after a few hours of changing a setting. I cant be restarting all devices every time I change something arround (I do some dnsflushing on my devices though). Any further help is much appreceated since I really want to get this to work. Me, I am also enjoing this since I consider it a hobby and I learn new stuff. My Family though would rather accept the adds then the internet outages. So I can only swith back to Pi-Hole beeing the main DNS Server on the LAN when I have figured it out.