If there was the reinstallation of OS - for example, Windows - due to such reasons as computer formatting, GOM Mix Pro Full version registration might be undone.
GOM Mix Pro, during installation, stores information on the PC to check period of using the Free version and Full version registration.
Information is deleted from the archive once the OS has been reinstalled, which seem to have caused cancellation of activation and return to Free version status.
In this case, please register again afterlicense deauthentication.
Please see here for instructions on how to deauthorize.