Webmail fails to load: "/usr/local/cpanel/base/webmail/paper_lantern/_assets/base.min.js: No such file or directory"

Symptoms

Webmail fails to load with a fatal error similar to one of the following:

CPANEL_ERROR: A fatal error or timeout occurred while processing this directive.
Template::Exception:
[TYPE]=[file]
[INFO]=[/usr/local/cpanel/base/webmail/paper_lantern/_assets/base.min.js: No such file or directory]

CPANEL_ERROR: A fatal error or timeout occurred while processing this directive. [close]
Template::Exception:
[TYPE]=[file]
[INFO]=[/usr/local/cpanel/base/webmail/x3/_assets/base.min.js: No such file or directory]

Cause

The cPanel account's package has a deprecated cPanel theme (paper_lantern or x3) assigned to it.

Resolution

1. Log in to WHM as root
2. Navigate to Home / Packages / Edit a Package
3. Select the package that has the deprecated theme assigned to it
4. Click the Edit button
5. Ensure that the cPanel Theme is set to jupiter
6. Click the Save Changes button at the bottom of the page