After version 1.3.6 of Apache, the semantics of the HeaderName feature (mon_autoindex) changed to be a relative URI and not a stub. Thus the configuration should have something like HeaderName HEADER.html and not HeaderName HEADER Likewise
I never finished the last sentence, after I went to read more Apache docs. The last sentence of my description should read "Likewise with ReadmeName"
We'll be fixing this for our next release (as of 1.3.14-12 and later). Thanks!