Answer
This has been traced to file and directory permission issues, affecting both the Windows-side folders/key files on the management server and the corresponding directories/files on the Linux middle-tier server — not a configuration mistake in main_config.json itself. Explicitly setting the correct permissions on both the Windows-side key/folder locations and the Linux-side SSH-related files and directories (rather than just the key file referenced in configuration) resolved this for at least one confirmed case; if the KEY step succeeds but KUBERNETES fails right after, permissions on both sides of that SSH trust relationship are the first thing worth checking.