also getting same error with new client 6.0
Are you running one of the precompiled wallets as per
https://www.bitcoinplus.org/wallet.php. In which case - which one ?
I'm not an expert on wallet errors but it sounds like a corrupt database. I would backup your wallet and delete the database file and let the wallet sync up again from the start.
If using Windows, the database is found in the folder - "c:\users\"yourusername"\appdata\roaming\bitcoinplus".
Sounds like the .blk got corrupted possibly by closing the wallet down too quickly or some other system issue.I tested all last 3 clients and no problems so looks like OS issue.