EXEXML Sensor cannot access Powershell AWS credentials

I have created an EXEXML Sensor on a remote probe which runs a powershell script on my Windows server. If I run the script from a CMD (using the SYSTEM account) it runs fine, but when I run it from PRTG, I get this error in the log file. In both instances, it should be attempting to use the IAM server role (no access keys involved).

get-sessendquota : Unable to retrieve credentials.
At C:\Program Files (x86)\PRTG Network Monitor\custom sensors\EXEXML\Custom 
Powershell AWS SES GetSendQuota.ps1:6 char:17
+ $sesSendQuota = get-sessendquota -Region "us-east-1"
+                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (Amazon.PowerShe...SendQuotaCm 
   dlet:GetSESSendQuotaCmdlet) [Get-SESSendQuota], InvalidOperationException
    + FullyQualifiedErrorId : Amazon.Runtime.AmazonServiceException,Amazon.Pow 
   erShell.Cmdlets.SES.GetSESSendQuotaCmdlet
 
<prtg>
<result>
<channel>Max 24 Hour Send</channel>
<value></value>
</result>
<result>
<channel>Max Send Rate</channel>
<value></value>
</result>
<result>
<channel>Sent Last 24 Hours</channel>
<value></value>
</result>
</prtg>

Here is my powershell script:

Import-Module AWS.Tools.SimpleEmail

$sesSendQuota = get-sessendquota -Region "us-east-1"
$Max24HourSend = $sesSendQuota.Max24HourSend
$MaxSendRate = $sesSendQuota.MaxSendRate
$SentLast24Hours = $sesSendQuota.SentLast24Hours

Write-Host "<prtg>"
Write-Host "<result>" 
Write-Host "<channel>Max 24 Hour Send</channel>" 
Write-Host "<value>$($Max24HourSend)</value>" 
Write-Host "</result>"

Write-Host "<result>" 
Write-Host "<channel>Max Send Rate</channel>" 
Write-Host "<value>$($MaxSendRate)</value>" 
Write-Host "</result>"

Write-Host "<result>" 
Write-Host "<channel>Sent Last 24 Hours</channel>" 
Write-Host "<value>$($SentLast24Hours)</value>" 
Write-Host "</result>"

Write-Host "</prtg>"

aws powershell prtg