There is going on more things between 6-7 am and its 'done' with server restart. Reason for kick is, that client (your PC) get updated market files and server still use 'old' till is restarted ... so if you do not restart game or do not log between 6-7, you can trade all the time to 'restart'. I can't explain why it is done such way, but obviously there are good reasons why. I will do note for myself to prepare wikis faq or troubleshooting with this info.