It sounds like you let MC25 generate a new Access Key on your Server for Clients to use for connections, and the Clients are still trying to use the old Access Key.
Find the last good working Library Backup of the previous version you used on your Server (MC23?). MC creates them automatically. Restore that backup on the Server. That should reinstate the previous Access Key, and Clients should connect again.
https://wiki.jriver.com/index.php/Library_Backup
I doubt that is the issue.
As I said in my previous post, upgrading to MC25 did not add an exception to the Windows Firewall, nor was I prompted to do so.
Going through the Media Network setup process in MC25 again, which kept the original access key,
did prompt me to make a firewall exception.
This only added an exception for public networks though, not private networks - which is the opposite of how it should be, so I had to change that myself manually.
Strangely, it seems to have since added
another entry for Media Center 25 access to public networks (I guess during a version update?) though it is disabled.
Hendrik said that he found the cause, but it has not been fixed:
https://yabb.jriver.com/interact/index.php/topic,115652.msg799220.html#msg799220