I have had Toolset registered.
Restored WP from backup pre - Toolset snapshot.
Now when attempt to register I get "Invalid site key for the current site."
Tried to unregister website and generate new site key again with www. and without - no change same error.
However could you send me the key that you are using so that I can try on my end. I was able to register the site with my support account but it needs to be registered under your account.
I honestly cannot say but since the only difference here is that I entered the key from my end it would mean that there is some incompatibility on your end.
Maybe it will work in a different browser for you but I still cannot give a definitive answer on this.