After upgrade to 5.0.2: "[HomeMenu] Discovery service error: " "{}"

We just upgraded our large RTC repository from version 5.0 to 5.0.2. Now, at least one user cannot use the Web-UI drop-down home menu any more.Instead, the following error message is displayed:

"A problem communicating with the Jazz Team Server discovery service has prevented the home menu from loading. Your Jazz Team Server may be configured incorrectly. Try again"

When looking at the logs captured by the browser, the following is logged:

"[HomeMenu] Discovery service error: " "{}"

To make matters worse, the drop-down home menu
- does not work if accessed from a project area.
- does not work if using Firefox (version 31.5.0 ESR on windows 7)
but, it
- does work if accessed from the user's personal dashboard.
- does work with alternative browsers (IE version 10.0.9200.17229, running on Windows 7)
- does work for other users

What can be done to fix this?

Thanks for your help,
Markus