From: Rabbi Moshe Sofer
To: Rabbi Yosef Yoel Deutsch
A man whose wife became mentally incompetent claimed she died and produced one witness to that effect, seeking permission to remarry. The Chasam Sofer rules strictly against permitting the remarriage, rejecting multiple arguments for leniency including that the cherem is merely rabbinic and that one witness suffices in rabbinic matters, and declines to be among those who permit.
Source: Shu"t Chasam Sofer EH I §2