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:
<staticContent> <mimeMap fileExtension=".mp4" mimeType="video/mpeg" /> </staticContent>