by Sergio Rabellino.
If I understood right, the upgrade is about the underlying operating system Windows Server 2012 -> 2019, an upgrade that broke moodle embedded files.
It's hard to say something useful, but if you're using an IIS web server, you should carefully check that all the configurations stay the same between the two OSes and double/triple check about file/directory permissions.
Anyway, without more precise information on this and some error from the webserver/php error log, who knows?