A couple of things to look at - Not sure if you're in an Exchange 2000 or 2003 environment.
1st, are you sure that you have the correct URL for OWA access? Another common format is: http(s)://mailservername.yourcompany.com/exchange
2nd, depending on the design of the forest/domain, you may have to authenticate with your username in the format: domain\username
I'd recommend logging in manually (using a standard web browser) first to be sure that you have the proper info to connect & authenticate.
There is also a possibility that your admins are smart and dropping all communication between your mailserver(s) and RIM's network. RIM thought they got cute when they added in the ability to connect OWA accounts via BIS. IIRC, i saw some comment by them saying something along the lines of: "Get your corporate exchange mail without your IT department". Yeah right.. Maybe if your IT department employs chimps!