According to The New York Times, the Boston Symphony said that she had canceled the Tanglewood concert and all of her other performances for the rest of the year for "personal reasons."
At the Tanglewood concert, she was to sing arias by Handel, as part of a program of Bach and Handel led by Harry Bicket. Violinist Corey Cerovsek is scheduled to appear as the soloist in Bach's Violin Concerto No. 2.
Her schedule also included performances of Mahler's Third Symphony with the Los Angeles Philharmonic on September 29 and 30 and October 1.
Bill Palant, Lieberson's manager, told the Times that Lieberson had few other engagements scheduled for the year. He declined to comment further, saying, "I don't want any inferences or implications to be made by anyone."
Lieberson has canceled a series of performances since January 2005 because of a back injury and a gall bladder obstruction. Among other events, she missed a recital tour, the world premiere run of John Adams's Doctor Atomic, planned appearances at the 2005 Tanglewood and Mostly Mozart festivals, and a tour with the San Francisco Symphony. She did appear to acclaim in several performances last year of Neruda Songs, a cycle written for her by her husband, Peter Lieberson.