OK, this may be one cause for the problem.
When I had installed Windows Home Server and the WHS connector utility, I had noticed that my shares menu under XP had changed. All of a sudden, there was a menu added to it in which I could set permissions on a user level. (usually I chose 'everyone'). Since a couple of weeks I have uninstalled the WHS connector software and installed XP SP3. I see I now have the 'old' Windows XP shares menu again, without the possibility of selecting permissions on a user level. (I know it used to be a separate tab)

Is this permissions tab something that needs to be activated separately in XP? If so, how?

My guess is this is the reason why Ubuntu refuses to connect to the share. It hasn't got the rights to, and for me, without that permissions tab, it's impossible to set them.

Anybody know how I can get that permissions tab back? I'm using XP Pro SP3.