Probably a hardware problem or bandwidth issue. Judging by the number of posts in JP forums to the posts in english forums (Not an exact science), we have a hair more than twice the players but we still have the same number of servers and probably the same bandwidth and physical servers. A mirrored setup is common when farms are in different physical locations. If you read the maintenance notice you'll see a disturbing line that reads
While we previously stated that we will be optimizing the servers prior to the emergency maintenance, we will require additional time to also implement counter measures to decrease the load on the servers.
which to me sounds like they will be doing further throttling of the number of players that can play.

There is a slight possibility that it's software but except for language it should be pretty much the same software.