JAVA created ical meeting doesn't show in outlook calendar when accepted

Question

With the iCal format, what could cause a meeting invite to successfully be sent to a user, but prevent that meeting from appearing in the user's calendar if it is accepted?

I'm working on a new feature in my java based web app, which integrates with outlook's calendar. User schedules something in my app -> calendar invite gets sent to their email.

I have one user who is testing the feature for me, and she reports that: If she uses my app to setup a meeting invite, the meeting does not appear in her outlook calendar when she accepts it.

She DOES get the calendar invite email, and can open and accept it, however it just doesn't appear in her calendar. This strikes me as odd...in my testing, outlook won't even open the invite if there is a problem with the ical formatting. This also leads me to believe that her issue is due to some setting in outlook that neither she nor I am aware of. She claims that she can accept other meeting invites and they appear in her calendar immediately.

She and I are using the same version of Outlook. I've got the iCal messages logged, and I can copy them into a text file, rename it to *.ics, and my outlook will open it just fine, and it appears on my calendar just fine.

I spent quite a bit of time searching around online, and I've got nothing.

Any ideas?