"The API request failed with the following error: 500 - Internal Server Error" when adding addon domains

Symptoms

Attempting to add an Addon Domain in cPanel fails with the following:

CPANEL_ERROR: Error: The API request failed with the following error: 500 - Internal Server Error.

Reviewing the cPanel error log, the following is observed:

CONFIG_TEXT: Connection Timed Out
Could not connect to https://203.0.113.2:2087/scripts2/getzones_local: Connection Timed Out at /usr/local/cpanel/3rdparty/perl/536/cpanel-lib/cPanel/PublicAPI.pm line 259.
Too many arguments for subroutine 'Cpanel::NameServer::Remote::determine_error_type' (got 3; expected 2) at /usr/local/cpanel/Cpanel/NameServer/Remote.pm line 41.

Cause

This error is typically caused by a DNS cluster server that is not responding. When adding an additional domain, all connected DNS cluster members must be accessible as various checks will be performed and will receive a copy of the DNS zone for the new domain.

Resolution

The server that is experiencing the communication error will be listed in the error found in the cPanel error log, and the connection will need to be reviewed further.

Where can I view the cPanel & WHM error logs?