Acronis Backup 12: logging in to the backup console fails for a user without a password

Symptoms

1. You install Acronis Backup 12 (on-premise deployment).
2. You try to log in to Acronis web console. The user you are trying to log in with is not password-protected. You provide the username and leave password blank.
3. Login fails with:
   The credentials are incorrect.

Cause

Blank passwords are not accepted.