Newer Windows, starting with Windows 7 at least, requires the clocks to be in sync for the login/authentication to work. The confusing part is that if it fails, it doesn’t tell you why, leading you to think your password was wrong.
Turns out this time, I’m trying to inject files to a Windows 2000 machine (a logic analyzer). After some Googling, this website showed me it could be a time issue. The RTC on that motherboard was alright, and showing that it’s 2018, but after a close look, the timezone was EST (GMT-7) while I’m on PST (GMT-8), so the clock is off by one hour!