There is a problem when we accidentally change cpanel hostname and submit it from WHM.
We will have a problem when we try to login into WHM or port 2086

This is because when we submit a new cpanel hostname, there is a intermittent / delay to cpanel license server to refresh the license and update cpanel new hostname.

To fixed this problem, we can just do it manually.
First, login into your cpanel server with root access

Type /usr/local/cpanel/cpkeyclt to refresh a cpanel license and re-sync it.