OK, Daron, the host was able to help. There’s an ‘Extra Web Security’ setting with Domain Management. Turning this off, then registering the plugin, has worked.The nice thing is that turning ‘Extra Web Security’ back on again doesn’t alter the ‘Valid’ value of the Licence Key (which now appears, as you were expecting).
So all good. Nice to have that tidied up.
Many thanks for your time and patience.