Japanese websites still do this. If you buy shinkansen tickets between 23:00-05:00 you get:

* JR central - you can buy, but not reserve seats, as the system actually registers your ticket at 5am

* Jr east - you cannot buy.

Some banks/government sites also simply don't want to work at night.

Very frustrating. To me it spells incompetence, but maybe there's some non-technical stakeholder who made a stupid rule once. For JR its probably some nightly accounting system - not 100% sure but I assume all the modern systems are just plugged on top of the ancient ones just like the airline industry does it.