What are the sins that are only forgiven by Hajj, as stated in the noble Hadith: "Among sins are sins that are not forgiven except by Hajj"?

Hajj is the fifth pillar of Islam and has great فضْل (merit), but there is nothing in the texts of Sharia indicating that there are sins that can only be expiated by Hajj. A hadith was narrated on the authority of Abu Hurayrah: "Indeed, there are sins that are not expiated except by standing at Arafah." This was mentioned by Al-Hafiz Al-Ajlouni, but its Iraqi source stated that he found no origin for it.