Rule#1, often quoted: Your Pi-hole has to be your only DNS server.
Don't define any other DNS servers, they will get used and by-pass Pi-hole eventually.
Your second nslookup from your RPi shows that Pi-hole is resolving correctly if used.
However, your RPi hosting Pi-hole is not using Pi-hole, which is ok. That's neither wrong nor right, your RPi may use any DNS server, see Correct Resolv.conf entry in pihole 5.11 - #2 by jfb.
If you want to verify your clients are using Pi-hole, run nslookup from a client.
You also have configured a potential DNS loop, having your router sending DNS queries to your upstream Pi-hole, while Pi-hole may send its queries back to your router at 192.168.178.1, as you've defined that as one of Pi-hole's upstream DNS servers. If used, a query will get passed back and forth forever, or until timeout.
It would be preferred if you'd configure Pi-hole as local DNS server as opposed to your current upstream setting, and your FritzBox router does support this.
If you do so, make sure your clients renew their DHCP leases so they will update their DNS server information. Dis- and reconnecting to the network should do the job, or powercycling a device.
For a start, have a look at Pi-Hole und FritzBox Setup Anleitung - #128 by Gert_Chlupaty (assuming you can read German). 