If you are experiencing this error it means that at some point you have added the license key to a Mister Horse account.
When a key is added to Mister Horse account it no longer work, however you can use your Mister Horse account instead.
To use your products with Mister Horse account do following:
1. Login into Animation Composer like this
2. Open "Manage Products" section.
3. Download your if necessary
4. The products should work from now on
If you cannot see your products in the Manage Products dialog then make sure you are using the right Mister Horse account.