The problem could be a number of possible things.
You could have malware which is messing with the clipboard and resulting in bad text being copied and pasted. In this case, you should perform a malware scan on your computer.
Your wallet file could also be corrupted so the passphrase no longer works (and also the private keys are corrupted so you may not even be able to access your Bitcoin). In this case, you should restore a backup of your wallet file (of course back up the wallet file first to another location, then restore a different, older backup) and trying to see if that works too.
You could have malware which is messing with the clipboard and resulting in bad text being copied and pasted. In this case, you should perform a malware scan on your computer.
Your wallet file could also be corrupted so the passphrase no longer works (and also the private keys are corrupted so you may not even be able to access your Bitcoin). In this case, you should restore a backup of your wallet file (of course back up the wallet file first to another location, then restore a different, older backup) and trying to see if that works too.
That sounds scary but I don't think this is the case because I manage to copy/paste from other text files just fine. The corrupt wallet is even worse. Is it normal for wallets to just lock like that? If so, I'd consider that a SERIOUS bug in Core client. Any documentation that you know of mentioning this as a common problem?
I found this error message on the Debug file and it never appeared at any time since I installed it. Oddly enough, it appears exactly on the day that my wallet stopped accepting my password:
2017-12-05 13:02:17 No rpcpassword set - using random cookie authentication
2017-12-05 13:02:17 Generated RPC authentication cookie C:\Users\USER\AppData\Roaming\Bitcoin\.cookie
What does it mean???
