I really don't know what went wrong there but you are very lucky to have such a kind pool operator that credited back your amount of funds. All I can advice here is to ALWAYS first send small amounts to your wallets first (especially if it's a new one) and then transfer the full amount after the first transaction got through.
I know, I have mined on suprnova since 2014 and always been very happy with ocminer, so I am not targeting the pool in any way.
I'm trying to get to the bottom of the issue, really. The pool does not allow you to withdraw a small amount to test with so I had to go with a full cash-out. Happy it got refunded...
Next I will cash-out to an exchange wallet and from there try to transfer a small amount to my local wallet again (I will delete the cache file first and see if it makes a difference). If that fails I'll have to create a new wallet and try again... and hope the operator over at nanopool will be as nice as ocminer...
@Febo: if the old keys are the cause of the problem, I assume I'll have no way around creating a new wallet. But this seems like the most plausible scenario/explanation.