In some rare occasions we have observed that a restart of Plesk services on the machine is necessary. Maybe the issue can be resolved by running
# service psa stopall
wait until all are stopped, then
# service psa start
to restart. Don't go for the "restart" parameter. It should be the same, but from my experience at least it makes a difference in memory and process management to run the two commands instead of one if a service is suffering hickups.