MikroTik

I have a problem/question that just came up from a customer. They connect to my network via the hotspot feature in Mikrotik. From there they connect to their Exchange server with Outlook Web Access. They will connect to their exchange server and enter their username and password from the pop-up but the problem is that the when they log in they will keep getting the pop-up to log in. Any location within the web access you navigate to wil require the log-in.
Would there be a firewall setting somewhere to stop this?

Jeff Kratka