An extended outage on the Clipper card system that became a major point of frustration for BART riders last month appears to have been caused by an account being shut off due to an unpaid bill.The rollout of the so-called "Clipper 2.0" system, which was meant to streamline fare-paying and allow transit riders to use their own credit cards to tap-to-pay, was called "a hot mess" by an executive board member of Clipper four months ago. And much of …