I had a similar problem, but it was the subwoofer instead of the center. I recently got a s5100 & have a Pioneer Elite AVR. It only happend once and it worked correctly after turning the AVR off and back on. I hope it's not going to happen again. Does your problem happen repeatedly? Any ideas on what could be causing it?
I also have the Pioneer VSX, but a bit of different problem than both of you. On mine, the sound cuts out intermittently during loud sequences. It happened last night while watching the Oblivion blu-ray. Any explosion scene or spaceship taking off resulted in cutting sound. Have you experienced this?