I was reminded today of a module I wrote a while back for Apache httpd 2.4 when I was debugging a config snippet from a customer and saw
[core:trace3] ... request.c(311): fixups hook gave 400: /
Whatever module set 400 (HTTP_BAD_REQUEST) didn’t log anything. If you have Apache httpd 2.4 and build it yourself, this type of issue can be solved with mod_hook_ar. This message from mod_hook_ar significantly shrank the search area:
[hook_ar:error] mod_rewrite.c fixups -> 400
Unfortunately, mod_hook_ar doesn’t currently have its own web page. You can download the code and information about building it from http://emptyhammock.com/downloads/, and you can read about it starting at slide 46 in this presentation: