Symptoms
Task of the type Set SPS info for user or Attach all users to SharePoint site fails with the error:
Destination host 'HOST001.providerdomain.tld' (#303), IP '10.1.1.10' : Provisioning request failed. Unknown error 0x81020054 [<response><errorContext description="The user does not exist or is not unique." code="0x81020054" executeSeqNo="3"><errorSource namespace="Error Provider" procedure="SetError"/><errorSource namespace="Error Provider Ex" procedure="RethrowError"/></errorContext></response>]
Cause
Incorrect User Account Directory Path in the SharePoint content database:
select Id, UserAccountDirectoryPath from Sites where HostHeader = 'lightingonthenet.collaborationhost.com';
B1F9F520-03B7-44E8-9C56-12A722F8CAB6 ou=Company Name,ou=Company2,OU=Hosting,DC=Provider,DC=tld
Here OU=Company Name is different from the company name in OA.
Resolution
Correct UserAccountDirectoryPath in the Sharepoint database manually and resubmit the task.