Shadow blur radius would have never occured to me from afar, the advantages of testing the actual object and some lateral thinking... Well done Bwldrd.

I don't think I've ever seen such a problem with cast shadows before, the only reason I know about the min bias and subdivision solution is via a long ago lecturing from Bagginsbill... But that wasn't a cast shadow thing but rather a direct object surface problem.

Being that you've already released the candle prop as a freebie and the overall solution is turning off shadow casting for the candle, maybe update your freebie with a different version? It's obviously the flame part of the prop that's the main culprit... So an update with the flame part separated as a parented part with it's shadow casting off, leaving the candle body as the shadow casting source, would seem to be best. That's what I would do at least.