Pi hole stops working after a few days

It looks like the majority of your DNS queries are coming from the Fritz Box IP, but they are unlikely to be originating with that device.

I do note that your DHCP server (the Fritz Box) appears to be passing out it's own IP for DNS, and not the IP of the Pi-hole:

*** [ DIAGNOSING ]: Discovering active DHCP servers (takes 10 seconds)
   Scanning all your interfaces for DHCP servers
   Timeout: 10 seconds
   
   * Received 548 bytes from eth0:192.168.88.1
     Offered IP address: 192.168.88.145
     Server IP address: 192.168.88.1
     Relay-agent IP address: N/A
     BOOTP server: (empty)
     BOOTP file: (empty)
     DHCP options:
      Message type: DHCPOFFER (2)
      server-identifier: 192.168.88.1
      lease-time: 864000 ( 10d )
      renewal-time: 432000 ( 5d )
      rebinding-time: 756000 ( 8d 18h )
      netmask: 255.255.255.0
      router: 192.168.88.1
      dns-server: 192.168.88.1
      domain-name: "fritz.box"
      broadcast: 192.168.88.255
      ntp-server: 192.168.88.1
      Port Control Protocol (PCP) server: 192.168.88.1
    
   DHCP packets received on interface lo: 0
   DHCP packets received on interface eth0: 1