Thanks for all the suggestions. I finally got the wallet unlocked. I am using the Linux Bitcoin Core CLI version. The problem was that I was using an ! in the password, which Ubuntu interpreted as a command, despite the password being encased in double quotes. So in the place of the !, Ubuntu suddenly began to interpret the password with "bitcoin-cli getblockcount" as part of the password instead of the !, which was apparently the prior command I had used before setting the password. The only way I was able to discover that bizarre result was because I had made note of a strange response from Ubuntu when I originally created the password. But I never had an issue after that, until just few days ago. The password worked with the ! for many months. It was only in the last few days that the password with the ! stopped working. That was a truly amazing string of coincidences. I am going to pray the rosary this weekend, in thanks, for sure.

By the way, my password has several special characters. I use quotes to enclose in Ubuntu, which always worked before.

Suddenly, I cannot unlock my Bitcoin Core encrypted wallet. I was sending and receiving transactions only a few days ago. I am certain that I am using the correct password because I had backups of the password, and it is correct. My bitcoin remain in the wallet, but I cannot move them. I moved the wallet.dat file to another computer, but I get the same error on using the password. Is it possible that the recent upgrade to v0.15.0.1 affected the password?