The root cause of this problem has been found : the HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders was missing. Problem solved.
However, this raises a problem regarding how the TreeView control in the Folders panel is programmed. On the affected system, all applications using the TreeView control were functioning normally. Only DPL had a problem. I did some monitoring during the DPL startup and I found that it was always trying to access the above mentioned registry key (Desktop value), probably to retrieve the user’s desktop path in order to provide the root item of the TreeView. If the key doesn’t exist, there’s obviously a problem.
Since the absence of the registry key was only affecting DPL, I’m wondering whether the code managing the TreeView control is retrieving the user’s Desktop path directly from the registry instead of using the relevant API. Windows has fallback mechanisms allowing to return a valid answer even if something is wrong in the registry. If only DPL fails when the registry key is missing, this seems to indicate that it tries to retrieve this information from the registry directly. After restoring that key with default values, DPL worked again normally.
So maybe a quick look at this code could be worthy.