Sorry, hit the post button early and was shortly thereafter called away.
Basically, I've got a more-or-less vanilla install on Raspbian Stretch and lighttpd was configured with mod_compress by default. This default (maybe not the fact that this module was enabled, but also the way it was configured) breaks the Query Log page of the web interface.
When I comment out mod_compress, the Query Log page displays log info. With the module active, it displays an error in the Dev Tools console: ERR_CONTENT_DECODING_FAILED and does not display any logging data.
Relevant portion of lighttpd.conf:
compress.cache-dir = "/var/cache/lighttpd/compress/"
compress.filetype = ( "application/javascript", "text/css", "text/html", "text/plain" )
Here's some of pihole -d:
*** [ DIAGNOSING ]: Core version
[i] Core: v4.3.1 (How do I update Pi-hole?)
[i] Branch: master
[i] Commit: v4.3.1-0-g1a741f6*** [ DIAGNOSING ]: Web version
[i] Web: v4.3 (How do I update Pi-hole?)
[i] Branch: master
[i] Commit: v4.3-0-g44aff72*** [ DIAGNOSING ]: FTL version
[✓] FTL: v4.3.1
*** [ DIAGNOSING ]: lighttpd version
[i] 1.4.45
*** [ DIAGNOSING ]: php version
[i] 7.0.33
*** [ DIAGNOSING ]: Operating system
[✓] Raspbian GNU/Linux 9 (stretch)