This issue has suddenly arisen for me again on the eve of the Steam launch after months of play with numerous other issues (all of which I've been able to fix). I have currently tried everything short of rebooting my system and/or reinstalling the game all over again in a different way. Thanks, SEGA and Microsoft. Your product up to now has been "flawless" without the login issues. I'm looking forward to seeing how you'll mess up the Steam release and lose a whole bunch of the install base on one day due to a similar, hypothetical login error, but seriously... just let me play the game. Stop messing with how the login credentials get processed, and just let me play.
Update: Upon rebooting my system, the game lets me log in with my previously working credetials. Thanks, Microsoft for making such a "flawless" product as Windows 10 and allowing me to play games like Phantasy Star Online 2 upon it equally as "flawlessly" on a monthly basis. I like gaming and fighting with my operating system in order to play games... If only I weren't lying through gritted teeth, you would know how much I "love" your products and support, but you don't care either way. Please fix your janky garbage.