Hi, thanks for the reply, but unfortunately it still doesn't work.
Ok... now this might sound REALLY stupid... but try setting the password (temporarily) to something ridiculously simple... like abc123 or password and see if you can use the 'walletpassphrase' command successfully...
If you have a long complicated password, it might just be that you're typing too fast and mis-timing a "shift+letter" for an uppercase or special character... I've done this quite a few times