Very tired now so I'm having a quick look and trying to understand the problem. Is it as simple as the time of two adjacent blocks being out of sequence, the more recent one having an older time, resulting in a negative number when calculating the last time period?
So somehow a block which based on timestamp should have been around #411235 ended up in the blockchain at #41228. The next block was the final PoW one.
That's enough to cause the network to get stuck?
I can see the same thing happened to CDC earlier this morning.
Is this a hard fork where EVERYONE will need to update their client in order to mine, or will it be sufficient for the majority of the network to update?
Yes - wrong timestamps of blocks - is a part of conditions to trigger bug. But PoS involved too - of no PoS blocks mined at same time - wrong PoW blocks will be just orphaned and situation selfcorrected after some time (few hours MAX). But with PoS block at same time (and without full check in PoS gen code) wrong PoW blocks are sealed in the blockchain and can not be orphaned by PoW miner (it can be orphaned by other PoS miners but they do not do so as PoS code does not see any problem in such blocks and regard them as valid)Code:
31-Dec-2013 13:56:33 height: 411227 mint: 0.25 difficulty: 0.17286038 flags: proof-of-work
31-Dec-2013 14:56:05 height: 411228 mint: 0.25 difficulty: 0.17353002 flags: proof-of-work
31-Dec-2013 13:58:21 height: 411229 mint: 0.25 difficulty: 0.05879355 flags: proof-of-work
31-Dec-2013 14:07:36 height: 411230 mint: 0 difficulty: 0.00024414 flags: proof-of-stake
31-Dec-2013 14:56:05 height: 411228 mint: 0.25 difficulty: 0.17353002 flags: proof-of-work
31-Dec-2013 13:58:21 height: 411229 mint: 0.25 difficulty: 0.05879355 flags: proof-of-work
31-Dec-2013 14:07:36 height: 411230 mint: 0 difficulty: 0.00024414 flags: proof-of-stake
So somehow a block which based on timestamp should have been around #411235 ended up in the blockchain at #41228. The next block was the final PoW one.
That's enough to cause the network to get stuck?
I can see the same thing happened to CDC earlier this morning.
Is this a hard fork where EVERYONE will need to update their client in order to mine, or will it be sufficient for the majority of the network to update?
About fork. Actually we have 2 options:
fork from 411227 block then only majority of miners(PoS miners mainly) need to update. After miners with updated client build longes chain (compared to stuck one) on all other(not updated) clients all blocks after 411227 will be orphaned and replaced by new chain without invalid blocks automatically. Without any soft updates.
And my first patch was this version - i just BAN (blacklist) block 411228 and rebuild blockchain - PoW mining start normal working from 411227 block.
But later I refused this option - because it creating a large number of orphan PoS blocks (which can include some real important transaction for the last 5-6 days) + this variant need a large number of PoS miners with large amounts of coins in stack to quickly generate at least 400-500 PoS blocks and overtake the current chain + this NOT prevent such bugs again in future.
In second patch i remove block ban and add simple check for negative time spacing. No need to orphan current blockchain, but all will need to update because old clients not accept blocks from new (even if it from longest chain). If not all update clients it will lead to blockchain fork to 2 parallel chains.
So it is some sort of hardfork now...
Don't hold much hope, over in the Stablecoin thread there have been about 4 new releases over the past 6 months, some peers are still running the original client! 
(it may include some real transactions) 
