There are some details here [Solved] trying to use Joplin Android App with lighttpd - Lighttpd - lighty labs
This is the essence of it as far as I understood
Does the resource in question exist? If the resource does not exist, and If-None-Match is sent spuriously by Joplin, there might be a logic bug in the lighttpd test with If-None-Match
And this as well
I confirmed that the patch above in #message-10194 fixes the 412 Precondition Failed response that lighttpd was returning to the random If-None-Match sent by Joplin for a non-existent resource. The patch is part of lighttpd 1.4.64, released Jan 2022.