After rewiring the DHCP relay scripts, the version file was forgotten.
This is what happened to my config after that: since version file was missing, the migration script runner decided the current version is 0. Then it found dhcp-relay@1 in the config version string and invoked the 1 to 0 backwards migration scripts from the old days, which migrated the config to an old syntax the current system doesn't understand, breaking the config loading.
@c-po Please do not forget version files. ;)