Yeah, this might have helped.

The solution I tried involved changing some files in /Library/Keychains, but it didn't work

And the worse thing is that after that, a disk scan (using the Recovery boot) showed some errors on the HD, but these were non-recoverable

If you boot into recovery drive and go under Utilities -> Terminal type 'resetpassword' and in that same window there will be a ACL ( OS X Access Control Lists ) reset for an account. If you do that when you reboot, it prompts you to make a new keychain before logging you in.