I still can't log in to my SPG account at all, even though I'm 100% sure I'm using the right password and I cleared my cache/used an incognito browsing window. It won't process a password reset either. It just either says the account doesn't exist, or returns "This service is temporarily unavailable. But don't worry, we're on it. Please try again later", depending on whether I use my old or new SPG number or my email address.
SPG/Marriott Support say their systems are still experiencing difficulties, but it sounds like I'm the only one with this problem!