I am facing the same problem. Some other people on reddit said they have the same error message and it ended up being an incorrect password. It is highly suspect that we are facing the exact same issue though. I have a wallet that I created in 0.5.14 that gives me the same error message when I entered in one of the main passwords I used.

Someone also told me there were messages in the code indicating that the way the wallet was encrypted changed between 0.5.14. and 0.5.15. It is also strange that they removed that version for download. Not saying that is a cover-up but I did want to try decrypting the wallet with that version in case by some miracle it is able to decrypt the keys/ wallet.

Are there other threads you know of with people facing this same issue?