If you are experiencing this error it means that at some point you added the license key to a Mister Horse account.
When a key is added to Mister Horse account it no longer works; however, you can use your Mister Horse account instead.
To use your products with your Mister Horse account, do the following:
1. Login to Animation Composer like this
2. Open the "Manage Products" section.
3. Download your products 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.