404 error message after login into cPanel

[George]

Some time When logging into cPanel, we are receiving the 404 error message can't find a theme (./frontend/bluelagoon/index.html) , the page can not be displayed.

This is more than likely because the theme is missing.

To get cPanel working again follow the steps.

Edit /var/cpanel/users/username and change the theme to default and save it.

It should bring up the default theme. If its not, then look in the
/usr/local/cpanel/base/frontend/ for available themes and change it to one that is there.

You can also install new themes from WHM as per your requirement.