Magix support sent this recommendation, which worked like a charm…
In the Registry Editor, navigate to the following directory:
HKEY_LOCAL_MACHINE > SOFTWARE > Wow6432Node > Sony Creative Software > [Program Name] > [Version Number] > License
In this case:
HKEY_LOCAL_MACHINE > SOFTWARE > Wow6432Node > Sony Creative Software > VEGAS Pro > 13.0 > License
Right-click the License folder ONLY and select Delete. If prompted, confirm you wish to delete the folder.