This might be the same problem as already mentioned in tickets #22 and #376. There it is mentioned as fixed.
I observed it with the following system:
Machine: Mac Mini Intel Core2Duo 1,83;
Host: Mac OS X Leopard;
Guest: Windows Vista x64 SP1;
Virtualbox Version 2.1.2;
I connected a drive to virtualbox shared folders. When I open it in Windows explorer, the drive symbol is shown with a red cross. But when I select the drive I can browse in the folder structure without any problems. Also programs can be started.
I recently installed a old setup program "called Wise". I opened there the file choose dialog in this program and try to select the drive. However, I got the following error message: An error occured while reconnecting S: to \\vboxsvr\development. Virtualbox Shared Folders: The request is not supported. This connection has not been supported. Seems that the old file choose dialog has a problem with mapped drives. Also, in the MS explorer, it shows the red cross (Not okay), but I am able to browse???