Plesk server hostname has
www prefix: www.example.com
Unable to add a subscription example.com:
This record will conflict with the glue record for the zone 'www.example.com'.
Also it is not possible to create a subscription with a name that contains Plesk server hostname.
This is an internal issue with ID #PPPM-1462 that has been fixed in Plesk 11.5 microupdate #36
Apply the latest Plesk microupdates.