Edit: just successfully downloaded 6.0.008 without a problem so I'll delete this in a while.
Before you install the new version - 6.0.006, be sure to copy your license key.
This is accessed by going to Help->Manage Licenses->Enter License.
Copy the text there into a suitable safe place.
When you install the latest RapidMiner Studio version, a nag screen will appear. You can escape this by entering the license text you carefully saved.
It turns out that if you run a stupid version of Internet Explorer (for me 8.* - I have no choice in this) then the license key does not show up on the RapidMiner site. This had the brief and tiresome side effect of locking me into a loop of despair.
With luck, this is a "swivel eyed mad feature" aka a "bug". If it is, I'll delete this post.