When you are trying to access CPane you will get below error.
Not Found The server was not able to find the document (./frontend/x3/index.html) you requested. Please check the url and try again. You might also want to report this error to your web hosting provider.
To fix this issue rename the below file.
mv -v /usr/local/cpanel/base/frontend/x3 /usr/local/cpanel/base/frontend/x3.backup
Then run upcp