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I had not heard of this happening until your post, but then again Windows 8 hasn't been around all that long. I poked around the interwebs a little, and found examples of a few people having similar issues, but no real definitive answer. I wonder if you could use a save point to revert back to when you don't have an issue, then reset your admin password as an open field to at least prevent you from being locked out. Even if it works you will have an admin profile with no p/w, which is obviously not ideal, but maybe it would give you time to find an actual solution.
I did find a post about conflict between Windows Live passwords and Windows 8 passwords due to the Windows 8 login only allowing 15 character passwords, but this obviously doesn't sound like it applies in your case since it actually worked for a bit. Other than running through the classic checklist (caps lock on/off, num lock off), I can't think of anything off the top of my head.
I wish I knew an answer for you, it must be mega-annoying. If you figure it out, post the solution, I would like to know what the issue is and how to resolve it for future reference.