Twain's representative recently confirmed the couple was splitting up after 14 years of marriage. The publicist did not offer details, however.
Sources close to the former couple said Lange was involved in an affair with a longtime secretary and manager of Twain and Lange's chateau in Switzerland.
"Mutt and (the secretary) left their spouses for each other and are still in a relationship," one insider told People.com.
However, Lange told the magazine, "It's not true," when asked if he was in a relationship with the secretary.
A source close to Twain said the singer is "devastated."
"This came out of left field... She loved him," the source said.
Justin Bieber crashes Drake Bell's album release party
Members of Congress to keep receiving porn magazine