I opened a case with PTC on this. They were not of much help, but I ended up figuring out a workaround.
All our config files have the line:
windows_browser_type mozilla_based_browser
After removing this line, the problem went away for the user. Firefox was installed on this particular machine, but perhaps it was an old version... or some other issue.
In any case, the reason this config option was set to begin with was because our reseller suggested it to improve performance with Windchill:
"The reason we usually recommend setting the browser to Mozilla in Wildfire 5 / Creo Elements Pro 5 is that with systems running Internet Explorer 7 or 8 it performs noticeably faster and is typically more stable with Windchill 10.1. In fact for many of our customers it corrects issues with lock-ups. If your systems were updated to Internet Explorer 9 or 10, then using Internet Explorer would be better, and is actually much better with IE 10. However, according to PTC IE 10 is only supported on Windows 8 systems."
Clearly in this case it was causing the lock-ups, so I have removed it and now everything works fine!
Andri