Parallels Plesk Panel key update deferral notification

[JessB]

Every day for the last 4 weeks or so at 07:25 I get the following email:

From: Administrator <webmaster@--domain omitted-->
Subject: Parallels Plesk Panel key update deferral notification
To: webmaster@--domain omitted--

Unable to update . An error occured while processing your key. You can try to update it later.

(I had to look at the mail headers to work out which of my Plesk servers was sending it!)

I've had emails with the same subject in the past, but the message itself only mentioned successful upgrade of the license key.

I hadn't done any upgrades or made any significant changes just prior to the emails starting.