When the SSTS tries to download a file that Globus does not have read permissions on, it returns an "Internal Server Error" rather than an error describing that permissions are not set properly. Change the service so that it returns a message stating that permissions do not allow downloading of the file.
Steps to recreate:
Copy a file to /simpleTransfer (or wherever SSTS is configured to upload/download files).
Change the permissions on the file so that the user that Globus runs as does not have read permissions.
Attempt downloading the file using the SSTS client.