In 99%+ of cases where a user reports "Kodi doesn't start" the OS is running happily in the background and if SSH has been enabled you can login via SSH to capture logs. If SSH is not enabled, drop 12.0 files in the 'Updates' Samba share and reboot to downgrade to the earlier release and enable SSH, then update to the problem version again.
Alternatively, boot from a 12.0 installer USB, interrupt boot and type 'run' to boot/run from USB instead of installing, then enable SSH to access the console and edit syslinux.cfg in the internal drive's boot partition (mounted under /var/media/) to append 'ssh' to kernel boot params so the SSH daemon is forced to start on boot. Then shutdown and reboot to the problem OS to poke around and share logs.
Or just do as suggested; access the 'Logfiles' share on 'a' problem box and upload the zip here. Until you share some evidence of the problem we have no idea what the issue might be, and cannot do anything more for you.