Dynamics (FetchXML) - Error: Aggregates are not supported - powershell

Question

I'm fairly new to the FetchXML / Dynamics scene so please excuse my ignorance.
Up until now I've been executing a PowerShell Script that runs a FetchXML query to obtain user most recent login dates within Dynamics.
This has been working flawlessly for about a month, but as of about a week ago the script stopped accepting my FetchXML query throwing an error saying "Aggregates are not supported".
No changes to the code have been made what so ever.
Weirdly, the exact same code worked in the UAT Dynamics environment but not in production. After a couple more days it appears that it's stopped working in the UAT environment too.
Any ideas as to why this could be occurring? I'll post the code below.
Thanks All
$xml = [string]::Format('
<?xml version="1.0" encoding="UTF-8"?>
<fetch mapping="logical" aggregate="true" version="1.0">
<entity name="audit">
<attribute name="createdon" alias="lastlogin" aggregate="max" />
<attribute name="auditid" alias="lastloginct" aggregate="count" />
<filter>
<condition attribute="operation" operator="eq" value="4" />
<condition attribute="createdon" operator="on-or-after" value="{0}" />
</filter>
<link-entity name="systemuser" from="systemuserid" to="objectid" alias="su" link-type="inner">
<filter>
<condition attribute="domainname" operator="eq" value="{1}" />
</filter>
<attribute name="fullname" alias="fullname" groupby="true" />
<attribute name="firstname" alias="FirstName" groupby="true" />
<attribute name="lastname" alias="LastName" groupby="true" />
<attribute name="domainname" alias="domainname" groupby="true" />
<attribute name="accessmode" alias="accessmode" groupby="true" />
<attribute name="isdisabled" alias="isdisabled" groupby="true" />
<attribute name="businessunitid" alias="businessunitid" groupby="true" />
<attribute name="internalemailaddress" alias="internalemailaddress" groupby="true" />
<attribute name="systemuserid" alias="ObjectID" groupby="true"/>
<attribute name="ms_provider" alias="Provider" groupby="true"/>
<attribute name="mobilephone" alias="Mobile" groupby="true"/>
</link-entity>
</entity>
</fetch>
', $date, $user.userprincipalname)