After installing the latest CU, that is KB3213986 on our 2016 Essentials box the server backup that night completed but did not back everything up that it should and was showing as green tick "incomplete".
A deeper delve into it, a rake of SQL database and server role stuff like DNS, DHCP etc, files and folders were not backed up. The tall and short of it was that as soon as the backup had started a healthy list of VSSAdmin List Writers was going from a healthy state to a failed / timed out state.
Having spent 3 solid days doing reboots, making registry tweaks, resetting the backup, running batch files to re-register .dll's the simple answer seems to be as follows....
There is a corrupt WMI state...file wmiutils.dll. (Assume following CU installation)
Fix: Open an elevated command prompt.
Execute: net stop winmgmt
regsvr32 wmiutils.dll
net start winmgmt
At this point, the Essentials console will mis-behave and report all sorts of weird things. A reboot is required and backups will return to normal.
I recently upgraded from essentials 2012 r2 to 2016. All went smoothly, except that the server backup wouldn't work. I used Martyn's solution and I'm now able to backup the server once again. Thank you!