Thanks Dave. The problem exists on only one keyboard. After it happened twice on the keyboard I was gigging with, I switched keyboards, but it is not repeating itself at home.

But when I had the problem for the first time, I checked levels, mute, and switched mic cable and mic, and the problem was still there. Fifteen minutes into the performance with no mic, I turned the keyboard off and on, and the problem went away.

The second time, turning off and on didn't help, but when I switched the keyboard display from non-mute to mute to non-mute, the problem went away.

Hasn't returned at home. It sucks when it's so elusive and non-repeatable.