Hi, thanks for replying.
I re-enable the voting plugin and it works again. 
I also tried installing the voting first and then qa, but it still works.
I would guess that it may something from the old qa and kept by voting leads the error even when deleting qa plugin. After deleting the voting to clean the data, the new qa and voting work together again.
However, I am not familar with those plugin interaction, so just guessing…
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