Is Blesta hosted on the same Plesk server?
Has been reported before that that doesn't work.
Plesk restarts the webserver and kills all running PHP processes -including the Blesta one- the moment you add or update any webhosting account.
Also concerns the same server that is hosting Blesta?
If it concerns multiple modules, Blesta should be changed to provision hosting packages asynchronously. (from cron, or by spawning a separate process that calls setsid() first so it doesn't belong to the same session as the webserver, and then performs the actions in the background)