Plesk requires addtional semaphores for it to run.

You can only do this at kernel boot time. I use the following and it seems to work:

You need to edit /boot/loader.conf

# semaphores for psa
kern.ipc.semmap="768"
kern.ipc.semmni="256"
kern.ipc.semmns="393216"
kern.ipc.semmnu="768"
kern.ipc.semmsl="1536"