I just downloaded 6.2.2a. I completely deleted all my temp aspnet files. I also uninstalled the prior version before installing 6.2.2a. The problem still happens for me. When I drag FROM a tree node, it's shows a drag image of the tree node. When I drag from anything else, it's a broken image red X icon. I also downloaded your latest catalog code. The SAME problem happens in the catalog code using the latest version of webgui. I don't know why it works on your online version but it looks like some resource or something isn't properly added.
I'm on XP SP3, Visual Studio 2008 SP1, .net 3.5 SP1. Cleared all my cache's, etc. Default theme. Vista theme.
The fact that the catalog I just downloaded stills shows a tree node image when dragging instead of the broken red X makes me thing that your latest change is not properly in the code?