Please follow the below template, it will help us to help you!
Expected Behaviour:
Admin should be able to migrate Pi-hole to another system and have it work.
Actual Behaviour:
Admin is a bit clueless. Pi-hole fails to work on the new system, until admin has a lightbulb moment.
This morning I spun up a new Raspberry Pi with the latest Raspbian Buster Lite from 26th September 2019, and installed Pi-hole on it. I copied across my existing custom dnsmasq configuration (/etc/dnsmasq.d/02-custom.conf) and custom hosts file which is referenced by that same 02-custom.conf file.
Pi-hole wouldn't start DNS, and the admin web page kept showing "Lost connection to API" and "DNS service not running", no matter how many times I tried to restart the FTL service or even reboot the Pi.
Eventually, a mental lightbulb illuminated and I looked in /var/log/pihole.log which revealed the answer: dnsmasq was refusing to load because a specific directory was missing.
I had been using dnsmasq to serve TFTP from the previous Pi. /etc/dnsmasq.d/02-custom.conf had an entry for /etc/tftp but I hadn't created the directory /etc/tftp on the new Pi. That's what was causing dnsmasq to fail.
Once I recreated /etc/tftp then pihole-FTL loaded successfully and DNS started working again.
If anyone else has similar issues after migration and was using a customised dnsmasq configuration before migration, check that all of the files and directories referenced by that custom configuration still exist.