The chat system crashes at 12:06 (just past midnight), no longer responding to input and becoming unavailable for a minute or two. After that you can log back in again.
You might be encountering the automatic server reboot - once a day the whole server reboots, which would affect chat/forum/etc.
I need to check the time this is set up - something like 4am would be a better choice....i have a sneaky feeling that it does occur around midnight/1am depending on how we are sitting with daylight saving time.