Hi Valerie,
The issue has been resolved. While the SPN for the service user SL-ABAP-FDV was indeed unique, closer analysis revealed that there was a duplicate SPN registered for our AS ABAP system (output below) which I remembered had been created while trying to configure SSO through another product some time back.
C:\Windows\system32>setspn -X
HTTP/basishost.xxxx.com is registered on these accounts:
CN=HTTP-basishost-xxxx-com,OU=Service Principals,OU=SAP Servers,DC=xxxx,DC=com
CN=sl-abap-fdv,OU=Service Accounts,OU=IT, DC=xxxx,DC=com
Removing the basishost-xxxx-com SPN enabled SSO to start working.
Many thanks to you and John for your advice and pointers. It is sincerely appreciated!
Kind regards,
Amer.
Note: For all who might see this post later, the command to check duplicate SPNs is setspn -X.