Are they accessing the database from web browsers or via Open Remote from filemaker Pro?
Given that this usually happens about the same time of day, have you tried checking logs and running network diagnostics to see if anything is going on at that time each day that might be a factor?
They are accessing via Open Remote. Yes, we have tried to check all the activities on the network and on the server during that time. We tried not to carry on some activities and observe what happens if we do this and do that. But still, the case is, one of our most important databases will not respond. Though others can be accessed, 2 Huge databases seem to crash and do not respond on that specific time. I've tried to google it and somehow, there are no clear solutions aside from rebooting the server, which waste precious time.