Just in the hopes of saving someone else the arduous journey that I went through. This turned out to be a permissions error. So. With Xymon 4.3.25 running on Ubuntu 16.04 server with Apache2 2.4.18, I was able to fix the 403 Forbidden error by editing the /etc/apache2/conf-available/xymon.cfg file and replacing every instance of Require local with Require all granted. After a restart of the apache2 server, and a restart of xymon, just to be on the safe side, everything worked out fine. The primary issue for me was that the xymon log files had errors, but following those errors never led me in the direction of determining that they were caused by permissions being used by Apache. If I had any hair, I'd have pulled it out by now.
Sponsored by https://www.newser.com/?utm_source=part&utm_medium=uol&utm_campaign=rss_tagl...
Police: Possible Plot to Attack Capitol Tomorrow http://thirdpartyoffers.juno.com/TGL3131/603fe9f7e370669f711f3st02vuc1 The Way Trump Called for Donations Raised Eyebrows http://thirdpartyoffers.juno.com/TGL3131/603fe9f812d3669f711f3st02vuc2 'It's Comical,' Woman Says of Outcome of KKK Flag Incident http://thirdpartyoffers.juno.com/TGL3131/603fe9f83622169f711f3st02vuc3