The callee (server(not server app)) is not available

I have a web app that features a pop-up calendar for date-picking and
several customers had sudden-onset of errors (and no calendar) when they
clicked the calendar. The javascript error is:

Error: The callee (server(not server app)) is not available and disappeared;
all connections are invalid. The call did not execute.

Any ideas what is causing this problem? (I've told them to disable pop-up
blockers already but they say there is still a problem)