Can not change scanner from "Plex Music Scanner" to "Plex Music"

Have tried thiswith several browsers (Edge, Firefox) directly on the server and also remotely - unfortunately with no success. I can change same parameters, like “concerts” or “local data”, but changeing the scanner is not possible.

BTW: Have moved my Plex from W2012 to a new W2019-server one week ago. Have copied the “plex directory” under the user to the new server. This worked very well.