labvirt1015-1018 are running the 4.4.0-83-generic kernel.
We have seen some REALLY BAD behavior from this kernel in labtest. Specifically, three hosts running that kernel lost all network connectivity when rebooted and had to be re-imaged. A more modern (-93) kernel didn't show this problem.
So, let's get those four hosts upgraded. 1015, 1017 and 1018 aren't currently holding any user-owned VMs, so they can be rebooted at any time. 1016 will require a scheduled and pre-announced reboot as it holds quite a lot of real, actively used VMs.