Note: I had issues with installing this, thankfully the client seems to work fine. I have included a tar file with the missing html files that don't seem to appear in /var/lib/xymon/www/. You can try skipping the line that cp *html /var/lib/xymon/www and see if it works for you! If not, just CP over the files. It will be a little weird, but you will see my home XYMon page for a few minutes till XYMon refreshes, then you will just see the XYMon host itself. Make sure /etc/default/xymon and /etc/xymon/hosts.cfg match!
This is one of those situations were I got it to work "somehow" (I think I Upgraded my previous installation perhaps, but fresh install I can't make work without this tar ball), and tried to replicate the process for this guide, but was UNABLE to. I figured out the difference (the html files that are in the tar ball), and that if I moved over the tar file from the working one to the new one, it would reset to default and work.
Install Server :
Make HOSTNAME4-HOSTS.CFG the same in xymon-client and hosts.cfg!
Redirect domain.com to domain.com/xymon (Only if this is the only website you host on this machine):