Pihole 5.14 in Hyper-V DNS Service is not running

[Bucking_Horn]

@Iosif_Zamfirescu

I replaced the second DNS server on my router with Cloudflare (instead of the router's IP address) thinking that this should provide me with a backup solution in case PiHole stops working (for various reasons

That may be sound while you sort your issues.
Just keep in mind that it is not a backup - your clients are free to pick any DNS server they are aware of for any individual DNS request at their own discretion. Pi-hole will be by-passed if it isn't the only DNS server.

@Iosif_Zamfirescu

Yes, my router stopped being chatty since I disabled CF and yes, I cleared the FRL.db that day due to space issues.
(...)
Like for example yesterday morning Pihole stopped working although the VM was ok (restartdns solved the issue).

In that case, it may be better to continue this as a separate topic, so we could mark this one as solved.

@Iosif_Zamfirescu

I did a test on https://www.test-ipv6.ro/ and I got a red message saying "Our tests show that you will have a broken or misconfigured IPv6 setup, and this will cause problems as web sites enable IPv6."

Your public IPv6 connectivity would be outside of Pi-hole's scope.
Pi-hole could be only involved here if it would block any domains that the tesing website would try to access, in which case How do I determine what domain an ad is coming from? could help to identify those domains.