Excel Service Errors

Set up excel web access in sharepoint, however, "An error has occurred. Please contact your system administrator if this problem persists. " keeps coming up when using internet URL. There is no such error when use intranet web address. Check the access mapping, the internet web address is using secure port, but in IIS, the "request secure channel" is unchecked. (interesting, the access mapping is not so strict). Re-enable the SSL channel to match the access mapping. Excel Web Access works back to normal.