Sorry for the issue that blocked the game for several hours during this night.
Unfortunately, the problem that caused it appeared around 2 or 3 am my local time, which obviously is not a time where I typically read e-mails. I noticed and confirmed the issue at around 7:30 when I woke up, and got it fixed at around 8:00 inbetween getting ready for work.
This specific problem should not happen anymore in the future, because I always try to eliminate the cause of the problem, and not just the specific issue.