It sounds like a errant applet may be to blame. While logged in as the user that experiences the problem, could you please go to the GroupWise Help menu, and select About Formativ. When the About box appears, select the configuration tab, then press the copy to clipboard button. Paste the contents of the clipboard into a new email message addressed to email@example.com. Once we receive the config information we may be able to detect the problem.
When you say the problem is reproducible on every machine tested, do you mean when the same user is logged in? You also mention other users work fine, so I assume the former.
I would also suggest you manually copy all the applets this user is accessing (the path will be shown in the configuration information referred to above) to another directory, then re-start GroupWise and see if the problem persists. If the problem dissapears it is likely one of the applets you moved was being executed on reply and is causing the problem. In this case you may also wish to ZIP all the applets this user has installed and sending them to the same address above for analysis.