I have to agree, i had the same problem... i've not had the time to try and fix... but if you don't get any joy here i'd suggest looking at the http://www.nagios-portal.org/wbb/index.php
forum... some great nagios people on there and your bound to find some help. Not saying no-one here knows the answer by any means... i've just found better answers from there regarding nagios in the past.
Its probably something to do with the fact they have gone to using main.php rather than what it used to be main.html.....
If you get an answer form the other forum... please post it here as well for all to see.
Its an annoying problem!