A bug in Plesk 12x prevents a server admin from setting subscription expiration parameters via the control panel. The remedy is to turn off the expiration for the sites on the server.
This is done by executing these three commands below as root:
#plesk db "update Limits set value='-1' where limit_name='expiration'"
#plesk db "update Limits set value='-1' where limit_name='expiration_soft'"
#plesk bin subscription --webspace-on