The virtual machine configuration could not be added. A configuration with this name already exists.
Luckily enough, I found Mohammed Adenwala’s problem adding a virtual machine in Virtual Server 2005 blog post which described my problem exactly. After deleting C:\Documents and Settings\All Users\Application Data\Microsoft\Virtual Server\Virtual Machines\virtualmachinename.lnk I was able to add the virtual machine to the administration website and run it.
Thanks for your comment! You saved me a lot of time!