Determined after a couple of hours of testing that a posting in the uvic events calendar contains the control character 0016 (synchronous idle). The constructor for the RSSReader in the hums site reads that in to a string and then attempts to parse the string into an XML structure. The parsing fails so the php process fails so the user sees an empty white page.
I added a line of code to the constructor which strips out all control characters (except carriage return and line feed) after the string is read in, but before it is parsed into XML. That seems to have solved the problem.
I wrote to Dave Wolowicz and suggested that the Rss output from the events calendar should be stripped of control characters that XML doesn't support before being shipped out to the client.