Can not start LXC container with error: unsupported Ubuntu version '18.04'

I restored an LXC container I have backed up from a newer Proxmox VE to an older node. To my surprise... it would not start. This is the error message I got when I debugged the LXC start-up process.


lxc pre-start with output: unsupported Ubuntu version '18.04'


When I press start button, it quickly fails and displays:




I thought it may be caused by file system (the LXC image) corruption. So I tried to mount and unmount it first using

pct mount 165
pct unmount 165

both worked perfectly so that was not the issue.

Then I made sure the file system is clean by running fsck against it, using this fsck command

pct fsck 165

That also ran perfectly clean!

After being puzzled, I researched and found people with similar issue, it turned out that Proxmox LXC start up script checks for version of Ubuntu and did not have the latest version 18.04 LTS Bionic Beaver.  So I updated this file:

nano /usr/share/perl5/PVE/LXC/Setup/Ubuntu.pm

look for this section:
my $known_versions = {
  
    '17.10' => 1, # artful
    '17.04' => 1, # zesty
    '16.10' => 1, # yakkety
    '16.04' => 1, # xenial
    '15.10' => 1, # wily
    '15.04' => 1, # vivid
    '14.04' => 1, # trusty LTS
    '12.04' => 1, # precise LTS
};

and replace it with this:
my $known_versions = {
    '22.04' => 1,
    '20.04' => 1,
    '18.04' => 1, # Bionic Beaver LTS
    '17.10' => 1, # artful
    '17.04' => 1, # zesty
    '16.10' => 1, # yakkety
    '16.04' => 1, # xenial
    '15.10' => 1, # wily
    '15.04' => 1, # vivid
    '14.04' => 1, # trusty LTS
    '12.04' => 1, # precise LTS
};


This SOLVED my issue and I was able to start the LXC container.

If you don't feel comfortable editing /usr/share/perl5/PVE/LXC/Setup/ 
You can also simply upgrade your Proxmox VE node to get the latest update from Proxmox.