can't start because bridge 'vmbr0' does not exist but it does

On one of my nodes the upgrade/change to ifupdown2 from ifupdown left the networking service no longer enabled at boot. May not be the case with you, but sudo systemctl is-enabled networking should return enabled. If not, sudo systemctl enable networking fixed it for me.
 
On one of my nodes the upgrade/change to ifupdown2 from ifupdown left the networking service no longer enabled at boot. May not be the case with you, but sudo systemctl is-enabled networking should return enabled. If not, sudo systemctl enable networking fixed it for me.

Sadly the issue keeps being there
 
For some reason when I rebooted my Thinkpad T520, I lost my IP address. dhclient brought it back but not vmbr0. Then thanks to @vesalius I restarted "networking" and then everything worked.
 

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