You might be having protection issues. I don’t use protected files, however I think that the way MTP might work is that if there are any MTP transferred files on the player then the player won’t connect to a different pc in MTP mode until after those MTP transferred files are deleted. I might be wrong about all MTP files on the player causing that, and perhaps it might only happen if protected MTP transferred files are on the player. You need to do some research on how protected files and MTP mode works.
If reinstalling Windows Media Player doesn’t solve your problem, then you might have to format the player(using the player’s menu) deleting all the files on it to get it to connect to a new pc in MTP mode. If you still have the old computer you could use that to connect to the player, and use the old pc to delete the MTP transferrred files(so you won’t have to format the player) so that the player could still have the MSC transferred files remain on it.
I tried using MTP mode for a brief time a few years ago, but since then I have stuck with MSC.
