The pallet_bridge_grandpa at Polkadot Bridge Hub is now in an inconsistent state, the Polkadot-GRANDPA-light-client running BH doesn’t correctly handle a corner case involving validator set changes following validator slashing. As a result of the bug, the GRANDPA light-client can fail to recognise the other side's validator set change and thus start refusing to accept subsequent validator set changes as well. More details can be found on this forum post
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We have encountered this bug on the BH, resulting in Kusama->Polkadot traffic being stuck. This upgrade fixes the bug so that it is protected from encountering the same issue.

Please note that there is a Polkadot runtime upgrade (to 1.2.0) already up for a vote, so the goal is to wait for that to happen before enacting a subsequent patch fix on top.

Also, be aware that this proposal is submitted on the Whitelisted Caller track - a parallel submission should be up for a vote on Fellowship to whitelist this proposal shortly.