I have the free and the premium versions activated.
The only other thing I did before this problem occurred was that a message kept showing in the plugin area telling me to install the premium version or cancel the license, I clicked on the"cancel license" hoping it would then allow me to clear the message.
All that happened was that the menu item disappeared and I then couldn't add the activation key any more.
I have tried deleting the plugins and re-installing them but that hasn't worked.