When you say they can't find the form to write back, what do you actually see. Is it a 404 error, blank window? etc.. What exactly is seen by one user and not the other?
Have you enabled UWC logging by setting the following in uwclogging.properties:
uwc.logging.enable= yes
uwc.log.level=FINE
Also have you checked the calendar servers http.log file which you can also set to debug level by setting the following in ics.conf:
logfile.loglevel="Debug"
These log files might show come clues as to what's going wrong.
Rgds,
Martin.