[Ethereum] Mining with geth on private network hangs indefinitely

I've got a private test network running as per this article. In particular my difficulty is set to 0x400; the entire genesis file I'm using can be seen here.

Mining with geth does not seem to work at all.

Here's what I've tried to do. First a fresh blockchain and account:

vagrant@debian-jessie:~$ mkdir stackexchange-example-chain
vagrant@debian-jessie:~$ geth --genesis local_genesis.json \
  --datadir stackexchange-example-chain \
  --networkid 9991 --nodiscover --maxpeers 0 account new

Then after dealing with that, booting up the console:

vagrant@debian-jessie:~$ geth --genesis local_genesis.json \
  --datadir stackexchange-example-chain \
  --networkid 9991 --nodiscover --maxpeers 0 console

And then what I understand to be the standard mining setup:

> eth.accounts
["0x699ec6d49641e59f65ba4bf72c52628059301e64"]
> var foo = eth.accounts[0];
undefined
> miner.setEtherbase(foo);
true
> miner.start(2);
true

The logs report that generating the DAG for epoch 0 finishes in ~1 second, and then the miner seems to hang. The longest I've let it run is about 15 minutes, but at the difficulty I've specified I understand that I should be mining plenty in far less time than that. If I check miner.hashrate at any point I just get a bizarre new BigNumber() not a number: [object Object] error.

I've also tried wiping the .ethash directory and generating a new DAG from scratch. Same result.

After stopping with miner.stop(2), a getBalance call verifies that no ether have been mined:

> eth.getBalance(foo);
0

If I call eth.getBlock('pending', true) I receive the following information (you can see the entire log/console session output in this gist):

{
  difficulty: 131072,
  extraData: "0xd783010400844765746887676f312e352e31856c696e7578",
  gasLimit: 268173313,
  gasUsed: 0,
  hash: null,
  logsBloom: null,
  miner: null,
  nonce: null,
  number: 1,
  parentHash: "0x522fe03765d5834422cd7cfc88c435f33bcd13d7a4c71cd8eaf321a8b3dd8ea3",
  receiptRoot: "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421",
  sha3Uncles: "0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347",
  size: 536,
  stateRoot: "0x2fa7b359e63faf5af52846537e67053ffd96d2fd33877704192c9c3e6e6266b9",
  timestamp: 1455530923,
  totalDifficulty: 0,
  transactions: [],
  transactionsRoot: "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421",
  uncles: []
}

Notice that the reported difficulty is much higher than what I've specified in my genesis file; 131072, whereas I assume it would be 1024. Maybe this isn't representing the same difficulty, though. Later on the totalDifficulty is reported as 0, perplexingly.

Does anyone have any insight into what could be going wrong here?

It seems like this is the same problem encountered in this question.