I ultimately gave up on dietpi and used Buster because of the unsupported OS error. It currently works for me right now but I'll have to figure out why pihole -g gives me a "DNS resolution is currently unavailable " error.
dietpi-software's Pi-hole install supports Bullseye for a long time, also as it sets the flag to ignore the OS error (so we're able to test things early). So OP used the official installer.
The official installer should btw work just fine on Bullseye with:
Lighttpd will show a warning about deprecated "compress" directives, but will use deflate as fallback, which is not shipped with Lighttpd core package anymore but is hence installed as separate package.
Opposed to this solution, next Pi-hole release has the compress directives removed from its Lighttpd config, so data is transferred uncompressed. But not relevant for a low traffic single user admin panel, where you're not bound to SEO or traffic rates, and where the other way round the little CPU time for compression is negligible .
Check on your pi, are there alerts?, is is connected?, has it lost complete access to WIFI?
4.Try to reinstall the raspberry pi OS and reinstall pi-hole
If these don't work, YIKES! you have to talk to someone else, this happened to me and I waited like 2 months and it started working again, hope this helps!