Re-installing Ubuntu 18.04 server solved the issue.
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bridge device is missing
Status in bridge-utils package in Ubuntu:
Ubuntu 18.04: Since last dist-upgrade of the bridge-utils (2018-03-21)
the bridge netwok device 'br0' is missing. A 'ls /sys/class/net'
prints the devices 'enp3s0 lo lxcbr0 virbr0 virbr0-nic' only.
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