Taskbar icon for Thunderbird displays as X
 

Hi,

I've updated my system recently to KDE 3.3.1-3.14 Red Hat (well, actually Scientific Linux CERN) and Thunderbird 1.5.0.5.
The icon for Thunderbird displays correctly in the panel but when I start Thunderbird, the icon appearing in the taskbar is a generic X.
Any idea how to solve that?

Thanks in advance,
Jean-Marc