Can you provide a little more information about the network the WLAN is connected to? Do you have access to the Firewall and / or any kind of NAC software that's running on that network? Have you tried pinging the Exchange server from a wired node on the network? Since it's not just happening on your device and the Blackberries are almost certainly hitting a BES server rather than going directly to Exchange I would expect something is blocking access to the Exchange server.