On the 30th of December, 2021, at 10:19 ET, a chain halt was reported. It turned out to be a fork.
Following the release of the updated Mainnet, consensus of witnesses fell below the required 66.6% threshold due to some nodes not updating in a timely fashion. Since the blockchain requires the nodes running to be similar in code, the discrepancies resulted in a fork.
There are no known bugs, however.
The mainnet has been restarted and witnesses are now re-syncing with everything operational.
On behalf of the witnesses, thank you for your patience and support.
UPDATE: The witnesses tried to bring back the network by setting a single isolated node. Unfortunately not all witnesses managed to collaborate. Because of this it became necessary to use some of the init witnesses to bring back the chain. Now additional nodes are syncing and coming up. The chain state and code remains the same as the last release.