What is the ruling on one who invokes illness upon himself if he does a certain thing, then wishes to return to doing that thing—is an expiation (kaffarah) obligatory upon him before proceeding, out of fear that his supplication will be answered?

No expiation is required for returning to an action you did not swear to abandon. However, if the action is forbidden, then it is not permissible to do it. One should not invoke curses upon oneself, one's children, or one's wealth, for the Messenger of Allah, peace and blessings be upon him, said: "Do not invoke curses upon yourselves, nor invoke curses upon your children, nor invoke curses upon your wealth. Lest you coincide with an hour when Allah is asked for a grant, and He answers your prayer."