There has to be something going on. SQL won't just arbitrarily decide to deny connections after 6:30pm because it is done working for the day. There has to be something happening. About all we can do from here is offer some suggestions about what to look for. Dig through the server logs and sql logs. See if there is a large amount of activity at that time. Maybe there is a sql job or a windows scheduled task? I am just throwing out ideas here...
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