Maybe a clue
Nov 6 03:40:43 virt3 pvedaemon[15570]: starting CT 104: UPID:virt3:00003CD2:00023167:5DC2870B:vzstart:104:root@pam:
Nov 6 03:40:43 virt3 pvedaemon[5406]: <root@pam> starting task UPID:virt3:00003CD2:00023167:5DC2870B:vzstart:104:root@pam:
Nov 6 03:40:43 virt3 systemd[1]: Starting PVE LXC Container: 104...
Nov 6 03:40:44 virt3 kernel: [ 1437.814376] EXT4-fs (dm-6): mounted filesystem with ordered data mode. Opts: (null)
Nov 6 03:40:44 virt3 kernel: [ 1438.058424] audit: type=1400 audit(1573029644.395:22): apparmor="STATUS" operation="profile_load" profile="/usr/bin/lxc-start" name="lxc-104_</var/lib/lxc>" pid=15612 comm="apparmor_parser"
Nov 6 03:40:44 virt3 systemd-udevd[15579]: link_config: autonegotiation is unset or enabled, the speed and duplex are not writable.
Nov 6 03:40:44 virt3 systemd-udevd[15579]: Using default interface naming scheme 'v240'.
Nov 6 03:40:44 virt3 systemd-udevd[15579]: Could not generate persistent MAC address for vethYK256A: No such file or directory
Nov 6 03:40:44 virt3 systemd-udevd[15578]: link_config: autonegotiation is unset or enabled, the speed and duplex are not writable.
Nov 6 03:40:44 virt3 systemd-udevd[15578]: Using default interface naming scheme 'v240'.
Nov 6 03:40:44 virt3 systemd-udevd[15579]: link_config: autonegotiation is unset or enabled, the speed and duplex are not writable.
Nov 6 03:40:44 virt3 systemd-udevd[15579]: Could not generate persistent MAC address for fwbr104i0: No such file or directory
Nov 6 03:40:44 virt3 systemd-udevd[15578]: link_config: autonegotiation is unset or enabled, the speed and duplex are not writable.
Nov 6 03:40:44 virt3 systemd-udevd[15574]: link_config: autonegotiation is unset or enabled, the speed and duplex are not writable.
Nov 6 03:40:44 virt3 systemd-udevd[15578]: Could not generate persistent MAC address for fwpr104p0: No such file or directory
Nov 6 03:40:44 virt3 systemd-udevd[15574]: Using default interface naming scheme 'v240'.
Nov 6 03:40:44 virt3 systemd-udevd[15574]: Could not generate persistent MAC address for fwln104i0: No such file or directory
Nov 6 03:40:44 virt3 kernel: [ 1438.436844] fwbr104i0: port 1(fwln104i0) entered blocking state
Nov 6 03:40:44 virt3 kernel: [ 1438.436847] fwbr104i0: port 1(fwln104i0) entered disabled state
Nov 6 03:40:44 virt3 kernel: [ 1438.436922] device fwln104i0 entered promiscuous mode
Nov 6 03:40:44 virt3 kernel: [ 1438.437036] fwbr104i0: port 1(fwln104i0) entered blocking state
Nov 6 03:40:44 virt3 kernel: [ 1438.437038] fwbr104i0: port 1(fwln104i0) entered forwarding state
Nov 6 03:40:44 virt3 kernel: [ 1438.440413] vmbr0: port 3(fwpr104p0) entered blocking state
Nov 6 03:40:44 virt3 kernel: [ 1438.440415] vmbr0: port 3(fwpr104p0) entered disabled state
Nov 6 03:40:44 virt3 kernel: [ 1438.440472] device fwpr104p0 entered promiscuous mode
Nov 6 03:40:44 virt3 kernel: [ 1438.440509] vmbr0: port 3(fwpr104p0) entered blocking state
Nov 6 03:40:44 virt3 kernel: [ 1438.440510] vmbr0: port 3(fwpr104p0) entered forwarding state
Nov 6 03:40:44 virt3 kernel: [ 1438.443486] fwbr104i0: port 2(veth104i0) entered blocking state
Nov 6 03:40:44 virt3 kernel: [ 1438.443488] fwbr104i0: port 2(veth104i0) entered disabled state
Nov 6 03:40:44 virt3 kernel: [ 1438.443546] device veth104i0 entered promiscuous mode
Nov 6 03:40:44 virt3 kernel: [ 1438.506753] eth0: renamed from vethYK256A
Nov 6 03:40:44 virt3 kernel: [ 1438.628176] audit: type=1400 audit(1573029644.963:23): apparmor="DENIED" operation="mount" info="failed flags match" error=-13 profile="/usr/bin/lxc-start" name="/proc/sys/kernel/random/boot_id" pid=15613 comm="lxc-start" srcname="/d
ev/.lxc-boot-id" flags="rw, bind"
Nov 6 03:40:44 virt3 systemd[1]: Started PVE LXC Container: 104.
Nov 6 03:40:44 virt3 pvedaemon[5406]: <root@pam> end task UPID:virt3:00003CD2:00023167:5DC2870B:vzstart:104:root@pam: OK
Nov 6 03:40:45 virt3 kernel: [ 1438.975772] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
Nov 6 03:40:45 virt3 kernel: [ 1438.975812] fwbr104i0: port 2(veth104i0) entered blocking state
Nov 6 03:40:45 virt3 kernel: [ 1438.975813] fwbr104i0: port 2(veth104i0) entered forwarding state
Nov 6 03:40:52 virt3 pvedaemon[5414]: <root@pam> successful auth for user 'root@pam'
Nov 6 03:41:00 virt3 systemd[1]: Starting Proxmox VE replication runner...
Nov 6 03:41:00 virt3 systemd[1]: pvesr.service: Succeeded.