Some time ago Bitcoin Core wallet versions 0.4 and 0.5 had a bug. You can read about it here and here. The core client might have left some private keys unencrypted when setting the password and hashing the private keys inside the wallet.dat file. Next versions had a fix and encrypted all the private keys.
Similar bug had Multibit wallet. The wallet software after setting the password left backup wallet files unencrypted.
Later versions of above software fixed that security holes and have encrypted all private keys and wallet files containing these keys. If you have your funds blocked and have old backups that had been not opened by newer wallet clients there is great chance for success recovery. Thru the years working in the crypto industry we have develop special software to help recover the private keys and the funds locked in them.
Please fill the contact form and describe your situation, your funds may be recoverable now!