NOTE: This issue is resolved, but posted here for anyone else that might have the problem.

We have our file distribution point on a Windows server using SMB. All information is filled out properly in the JSS, including Share Name, Workgroup/Domain, Port, and all the account info. Casper Admin will not connect; however, connecting via Go->Connect to Server... works fine for both read-only and read-write accounts.

Turns out that specifying the Workgroup/Domain somehow kept Casper Admin from connecting; in our installation, apparently this information is unnecessary for proper connection (YMMV). Clearing this field worked around the problem.