Ok, looks like I understood the reason. When panel renders status of update installation process, it performs request to the server, and sometimes this request appears in the very same moment, when backend service plesksrv is restarted. Normally, restart of service performed very fast and you shouldn't see any errors. Probably in your environment this procedure lasts a bit longer, and panel requests status of update in that moment.
Please don't worry and consider this problem as an issue in UI representation of update status, this is not critical.
Thank you for reporting this issue.