I've just purchased the plugin and it worked automatically in the 1st site, but on this I keep getting same Invalid site key for the current site message.
I've tried deleting and adding again - didn't work.
Tried with & without www.
Could you try generating the site key by going to Plugins -> Add New -> Commercial and clicking the register button then click on the "your account" url.
This should generate the site key for the exact url of the site.