All in 7xWS2016 servers all running Sophos were affected, across two of my clients.
Windows Defender was thus uninstalled from 3xservers with small C-Drives to prevent 0% free space issue.
Cause seems to of been a MSFT Windows Defender update for which a fix was released late last week.
Windows Defender has been re-installed on the 3xservers it was previously uninstalled from.
None of the 7xWS2016 servers are showing a repeat of the issue so assume MSFT fixed it with a Windows Defender update.