Conflicting MIME types between server-level and site-level IIS configurations.
If you are using custom web.config you should check that you are not loaded MIME type, which has already been inherited from server-wide MIME types settings.
To check it you should open "MIME types" for migrated domain, if your web.config has conflicting records, you will get an error like:
"Cannot add duplicate collection entry of type 'mimeMap' with unique key attribute 'fileExtensions' set to '.mp4'" .
You can fix this issue, by removing next entries from web.config file in site directory:
<mimeMap fileExtension=".mp4" mimeType="video/mpeg" />