We caught up on our Foreman upgrades and just ran to this. We noticed that we had older initrd & vmlinuz files, and they were all the same size.
We copied the good files over the bad files and then changed the permissions of the files so that Foreman couldn’t overwrite them again, and now the installer seems to work.
root@foreman:/var/lib/tftpboot/boot# ls -latr
...
-rw-r--r-- 1 foreman-proxy foreman-proxy 11657976 Apr 21 2020 ubuntu-mirror-c5UKFnM0aX6y-linux
-rw-r--r-- 1 foreman-proxy foreman-proxy 51733440 Apr 21 2020 ubuntu-mirror-c5UKFnM0aX6y-initrd.gz
-rw-r--r-- 1 foreman-proxy foreman-proxy 11657976 Apr 21 2020 ubuntu-mirror-LHD1xmkmftXG-linux
-rw-r--r-- 1 foreman-proxy foreman-proxy 51733440 Apr 21 2020 ubuntu-mirror-LHD1xmkmftXG-initrd.gz
drwxr-xr-x 11 root nogroup 4096 May 8 14:58 ..
-rw-r--r-- 1 stefanl sysadmin 51733440 Jul 7 14:07 ubuntu-mirror-rf32u3HGTMZf-initrd
-rw-r--r-- 1 stefanl sysadmin 11657976 Jul 7 14:07 ubuntu-mirror-rf32u3HGTMZf-vmlinuz
root@foreman:/var/lib/tftpboot/boot#
Foreman-proxy complains in the logfile, but I can live with that as a workaround.