Install-ADDSDomainController Credential Error 12 - powershell

Question

Hello,
I am trying to automate the installation and promotion of domain controllers, which are being added to an existing root forest with no child domains. I'm using the following code to perform this action during a ConfigMgr Task Sequence (so the script itself is being run as System):
function Install-DomainController
{
param(
[string]$CUST_DSRMPassword,
[System.Management.Automation.PSCredential]$CUST_ADDSCredentials
)
Write-LogEntry -Type Information -Message "Promoting the Server to a Domain Controller..."
try
{
Install-ADDSDomainController `
-NoGlobalCatalog:$false `
-CreateDNSDelegation:$false `
-CriticalReplicationOnly:$false `
-Credential $CUST_ADDSCredentials `
-Adprepcredential $CUST_ADDSCredentials `
-DatabasePath "$NTDSPath" `
-DomainName "$DomainName" `
-InstallDNS:$true `
-LogPath "$NTDSPath" `
-ReplicationSourceDC "$CUST_DomainControllerToUse" `
-SysvolPath "$SysvolPath" `
-Force:$true `
-NoRebootOnCompletion:$true `
-SafeModeAdministratorPassword (ConvertTo-SecureString -AsPlainText $CUST_DSRMPassword -Force)
}
catch
{
$ErrorMessage = $_.Exception.Message
$FailedItem = $_.Exception.ItemName
Write-LogEntry -Type Error -Message "The Item '$FailedItem' caused the following error: '$ErrorMessage'!"
}
}
The credentials are being passed as a parameter formatted as [System.Management.Automation.PSCredential]. I can verify the credentials are correct by opening a cmd window, running powershell and using the following code during runtime:
$CUST_ADDSCredentials.Password
$CUST_ADDSCredentials.GetNetworkCredential()
$CUST_ADDSCredentials.GetNetworkCredential().Password
And I know the user I'm passing on to the cmdlet is infact a member of the domain admins, enterprise admins and schema admins. Yet I still recieve the following error message:
User is not DA or EA. VerifyUserCredentialPermissions error message:
You have not supplied user credentials that belong to the Domain
Admins group or the Enterprise Admins group. The installation may fail
with an access denied error. Do you want to continue?
I'm puzzled as to what I'm doing wrong here. Would I perhaps be required to run the cmdlet in a different context by opening a seperate PSSession and using Invoke-Command -Session -ScriptBlock{}? Or is there perhaps something obvious I'm missing?
Thankful for any help :-)
Best Regards,
Fred