Apply Drivers - AdminService CMG

[Amg399]

We're in the process of testing applying drivers on external clients, using Adminservice over the CMG. Currently using version 4.2.1 of Invoke-CMApplyDriverPackage.ps1. When testing on an external client, the process fails to retrieve a list of available packages.
ApplyDriverPackage.log ends with two errors:

Failed to retrieve available package items from AdminService endpoint. Error Message: The remote server returned an error (500) Internal Server Error.
An error occurred while calling AdminService for a list of avalable driver packages. Error message: InnerTerminatingFailure.

Trying to narrow down where the issue is. Curious if anyone has a recommended alternate method outside of the Invoke-CMApplyDriverPackage script, of verifying Adminservice functionality over CMG?

[Amg399]

Curious if anyone had any insight on this? Any help would be appreciated.