There are couple things I suspect could be causing this issue
1. Video files: Your video file should be valid and supported, if you have custom video files you need to register them
2. Metadata name space: As per the documentation, it mentions custom namespaces must be registered
Please review following section from https://docs.adobe.com/docs/en/cq/5-5/dam/how_to_edit_metadata.html
Creating New Metadata Property for Assets
Note Custom namespaces (like myCompany for metadata myCompany:description) must be registered in the repository/CRX. Otherwise, saving the metadata form results in an error.
There is a step by step example for customizing image dam, please review following
http://therealcq.blogspot.com/2012/10/customizing-dam-image-metadata.html
For videos it could be similiar