I was using the popular-ish Metro skin for Steam (not sure if it's related), and a couple of days ago, once Steam started on Windows boot, it just prompted me to relog with my creds.
Red flag for it to happen all of a sudden, but I pressed on.
Signed in, no red alerts in my e-mail or anything, even to this day, and everything was, and is, okay... everything except all Steam client, and per-game, settings had been flushed (settings like "Steam Overlay enabled/disabled" for each game, and the throttle speed for downloads were reset, but game installations and library locations were intact), and Steam just insisted to redownload
all the Workshop content I was subscribed to, for installed games.
I just blamed it on random Valveness; I didn't know that there apparently existed a widespread issue.
Finally, I did revert back to the default skin, since Metro apparently got its UI elements all misaligned this time, and that was the end of this chapter.
For the record, other than the Metro skin, I was using an untampered, latest stable build of the client - new UI and everything.
Running smoothly since the "flush".
Hope you figure everything out, man. Bummers are called that for a reason.