When provider tried to load a private key using the "Load Private Key" button in the System > Settings > Encryption Keys of a Provider Billing Control Panel, the key was dropped instead.
A user that made an attempt to load the private key used Microsoft Internet Explorer with enabled "Compatibility" mode.
According to KB 112792, compatibility mode in Internet Explorer is not supported and should be turned off.
Turn off the compatibility mode in IE. Also it is possible to use another browser from the list of compatible ones described in KB 112792.