Perhaps you forgot to quit Plex server from running, during the copy process.
This usually results in a corrupt database.
Stop both servers, old and new and copy over the primary database file once again.
Location is mentioned in here: https://support.plex.tv/articles/201100678-repair-a-corrupt-database/
Another idea: if the Plex data folder is not in the default location on the new machine, you must be very careful with what you put into
Settings - Server - General - ‘Show Advanced’ - “The path where local application data is stored”
The path must not end with Plex Media Server
see Moving Metadata on Windows for more details.
