The Rebbe expresses surprise at the custom of voluntary fasting, urging instead the Baal Shem Tov’s approach of serving Hashem with health and joy. He recommends replacing fasts with refraining from idle talk or indulgent foods, following the advice of the Rebbe Maharash, and suggests a hatarat neder if the practice was repeated.
https://www.torahrecordings.com/rebbe/igroskodesh/015/009/5489