Sounds like you are not quite monitoring the website(s) you hope you are monitoring. Have you tried some of the suggestions in the comments for this ZenPack (at the URL you listed above)?
It is actually the Zenoss zencommand daemon that runs this monitor for you so the suggestion to run zencommand from the command line (as the zenoss user), should provide some helpful debugging. Eg for a device called fred, you would have:
zencommand run -v 10 -d fred
Inspect the output closely (depending on what other monitoring you have implemented, zencommand may be running other stuff as well as this http monitor).
Cheers,
Jane