Sounds like the answer for u is right there, it looks like u want too upgrade from anything backup lower than from v0.30 directly to v0.42b and that is not supportet
read this: Mail-in-a-Box version v0.40 and moving to Ubuntu 18.04
In quick :
U must first upgrade to v0.30 on the old box , than force a backup, and than go to the new machine with the ubuntu 18 for the installation and than restore of the v0.30 backup before rerun the setup
In your Case:
So with the old machine not existing i only see the option to save the backuped files,… run a old ubuntu, install MiaB (should be a v0.30) , do a restore of the backup u have , force a new backup (than should do a v0.30 backup u can upgrade with) , and save it.
switch to a ubunto18 machine, install MiaB, restore the new forced backup new backup from v0.30 and rerun setup