Every hour a lot of N/A replies, what is the cause?

I'm not sure this is even a Pi-Hole problem, but since it's most apparent there I hoped someone here can point me in a direction to solve the problem.

Every hour from 10 too till about 15 minutes past Pi-Hole has a lot of N/A replies for DNS lookups. At that moment internet slows down, pages load for 10 or 15 seconds or sometimes not at all. When running dig on the host of Pi-Hole it shows that it can't connect to my router. Pi-Hole is hosted as a container on a Intel NUC with Proxmox. The router is a unifi security gateway. As DNS resolvers I use Quad9 and dns0.eu. Pinging IP addresses keep working during this period. Resolving/visiting local domainnames (like pihole or my NAS) keep working.

Where can I start to look for a solution? It almost feels like a cronjob of some sorts, but there is not a lot of traffic going through the network at these times. Also the proxmox host does nothing at that moment.