Gracias Carlos.
Adding the SAPService<SID> user to the credentials PSE fixed the issue. I had added the <SID>adm user to the server and client PSEs. Adding the SAPService<SID> user to the client PSE fixed my particular issue.
sapgenpse seclogin -p <patch to PSE> -x <pin> -O <domain\SAPService<SID>>
and then restarted WDP.