There are two working sets - pool's working set and miner's local working set. The "unknown-work" message means that work doesn't exist in pool's working set anymore (or never existed). Pool working set cleanup procedure launched for every new block, received by pool from p2p network. Local working set cleared when miner receives "New work found" message from pool.

So, if miner got reject with "unknown-work" message, this means that local working set wasn't updated properly while pool's working set updated already. This could be caused by network problem, and can be resolved by manual expiry time decreasing. Try -E 30 option for cgminer/bfgminer.

P.S. Minimum shares difficulty border now is fixed at x64.