Does expiation become obligatory for one who forbade traveling with a group due to hardship and boredom, then traveled with them after these reasons ceased to exist?

The correct view is that the phrase "haram upon me" (الحرام علي), if it does not relate to one's wife and is not in one of the known forms of oaths, then there is nothing in it. If a responsible person (mukallaf) declares something lawful that Allah has made lawful for him as unlawful upon himself, such as food, drink, or clothing, it does not become unlawful for him, because the one who makes things lawful and unlawful is Allah Almighty. Some scholars, such as the Hanbalis and Hanafis, disagreed, considering such phrases to be an oath that requires expiation (kaffara) if broken. However, the most preponderant and authoritative view for us is what has been mentioned. You are not required to pay expiation even if you were to adopt the second view, as long as the reason that led you to take the oath has ceased to exist.