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Witnesses are incapable of producing blocks b/c there is an issue that prevents verifying the previous chain - a particular transaction is fouling up the works. Dan Larimer and witnesses are working the problem as fast as possible.

Update 5 (15:05 UTC)

Ryan R. Fox (Fox):
The BitShares blockchain remains halted. The Core Development Team continues to actively identify the root cause. Witnesses are available to implement the resulting patch when it becomes available. Please expect another update in nearly 30 minutes.