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If you are Experiencing issues with a Pi-hole install that has non-standard elements (e.g you are using nginx instead of lighttpd, or there is some other aspect of your install that is customised) - please use the Community Help category.
Expected Behaviour:
Without any changes from my end, PI-Hole started to not resolve or resolve very slowly all DNS lookups. I tried turning off and on DNSsec validation to see if it helped. I made sure I updated the rasp pi and the pi.hole to the latest versions also. Seems not to have done anything. Can you check log output for anything interesting or wrong? I rebooted the rasp pi a few times. I powered it off, pulling power out also just to be sure, no help.
Actual Behaviour:
DNS Resolution has not been an issue. When devices are configured with same Quad9 ip's for direct lookup, it works fine.