Foreman 2.0 is quite old version. While this issue does not seem to be Foreman related, I’d still start first with upgrading your instance to 2.5 or 3.0.
This error typically means that it booted from the network but failed to download next stage files. Double check the content of /var/lib/tftpboot/boot and see if the files there has a correct size/checksum. Another common cause is some DNS misconfiguration, the timeout typically indicates that. E.g. your provisioned host can’t reach the Foreman URL because it can’t resolve the IP.
If the files have 0 size, most likely your installation media is incorrect. The Smart Proxy tries to download these files when a host is switched to the build mode. If the URL is invalid, the result is invalid initrd and vmlinuz. You can always delete these files and rebuild the host (cancel build, build again). That will trigger download of these files from the installation medium defined for the Host in Foreman. So I’d start first with making sure the Smart Proxy can download the right files.