Deathandtaxes - in answer to your question, I entered the private key that corresponded to the address ending in Ycm from the pywallet output (i.e. the private key ending in ek I mentioned previously) into blockchain.info. That resulted in an address ending in UyHf, which did not correspond to any address in the pywallet output. So the secret keys in the pywallet output, while valid (in terms of length etc), don't seem to correspond to the addresses in that file at all - I say this because:

1) I picked an address - the Ycm one - and tried importing it with each of the secret keys in that file and electrum client indicated that none of the combinations were valid, and

2) I picked a secret key - the one that appeared to correspond the Ycm address and that ended in ek, and generated the corresponding address in blockchain.info, and it didn't correspond to any of the addresses in the output of pywallet)

I'm not sure whether this is a version incompatibility between pywallet and older versions of the client software, or a bug in pywallet, or some other issue (genuine corruption in the wallet.dat), but it is quite bizarre!

Stephen Gornick - that's an interesting link and describes what seems to be exactly the same problem. If anyone could shed any light on why this occurs (or how it can be fixed) I'm sure this could be of huge benefit to others.

mufa23, you're my hero

For everyone else's benefit - all of the bitcoins from my corrupted wallet were transferred by mufa23 to a new wallet I specified, completely untouched.

mufa23 - if you would be able to provide any color on how you fixed this, I'm sure it would be greatly appreciated by the community. My old wallet is completely redundant now, so feel free to post extracts from it (e.g. output from pywallet) if it helps explain (of course from my perspective you have done enough, but I'm sure others would greatly appreciate it). As per the PM I sent you, I'd love to send you a small token to express my gratitude.

Thanks again!

Thanks for your reply Deathandtaxes

Based on the output from pywallet, I tried importing the private key that corresponded to the right address into blockchain.info and it imported, but gave a different address. So unfortunately, while they are valid, they don't correspond to the addresses that they appear to correspond to.
    "defaultkey": "1HxxxxxYcm",  [this is 34 characters long in reality]
    "keys": [
        {
[lots and lots of keys]
  "addr": "1HxxxxxYcm",  [this is 34 characters long in reality]
            "hexsec": "xxxxxxxxxxxxxxxxxxxxxxxxxx", [this is very long]
            "label": "",
            "sec": "5Kxxxxxxek" [this is 51 characters long in reality]

[lots more keys]
[some other stuff]

When I enter 5Kxxxek as the secret key into blockchain.info, I get a new address that doesn't end with Ycm

Also, the electrum client allows you to import a keypair in plain text in the format address:key. I tried every combination of 1HxxxxxYcm and the sec entry, e.g.
1HxxxxxYcm:5Kxxxxxxek
1HxxxxxYcm:[other versions of 5Kxxxxxxek paired with other addresses]

in all of those, the electrum client said that the address didn't match the private key

Thanks very much for your offer to help - I'm PMing you now

VPN subscription - mullvad.net takes bitcoins (and has a zero logging policy)

I think have managed to corrupt my wallet.dat. I was initially running coderr's patched client on linux (ubuntu 12.04), but the most recent version was 0.5.3 (there is no updated version of the patched version). I was running bitcoind (as opposed to bitcoinqt). I received some bitcoins (almost 7 of them) at the address that was displayed by bitcoind in the javascript rpc client, before all of the blockchains were downloaded.

As more of the blockchains started downloading, I noticed that bitcoind started crashing. I figured it might be a software version issue, and upgraded to bitcoin 0.6.3. This version kept crashing so I backed up my wallet and rolled back to a fresh install of 0.5.3 on a separate (windows) machine, and tried to use the new install with the old wallet files (original backup and latest version) after the blockchain was fully downloaded. The client would crash saying the wallet was corrupted, or would load a different bitcoin address. I also tried versions of the wallet that had been run through bitcointools' fixwallet.py but could not get rid of the corruption.

I have also tried pywallet to get a text file with the keys. This produces output in a format as described in the forums, but the private keys produced by pywallet don't actually match the address (I tried entering the resulting output address:seckey pair into electrum, and it said they were incorrect). I even tried pairing the address with every seckey from the pywallet output, one by one, and electrum rejected all of the pairs. I can only assume that changing the version of bitcoin, and/or switching from the coderr "enhanced anonymity" version of bitcoin to the official client corrupted my wallet.dat.

It is remotely possible that the wallet.dat is not actually corrupted, but is just consistent with the old 0.5.3 coderr version of the client (which incidentally cannot be used to export private keys - that was only introduced in 0.6) and the new version of the client (and the older windows client) just see it as different, and assume it's corrupted (and in the case of pywallet, maybe it misinterprets it). This would be consistent with the fact that I have also tried starting a new install of coderr's 0.5.3 on linux with a newly generated wallet.dat, and I still run into the issue that at a certain point, it keeps crashing as it tries to download the blockchain.

However, I'm at my wit's end, and would greatly appreciate help from a "Hero member" as I don't think there's much more I can do on my own now, so I will have to trust someone with my wallet.dat.

Please let me know if you would be willing to help

Many thanks!