Adding attribute AnonymousAdminLookupEnabled="false" in the SecurityConfiguration tag of WebLogic 8.1 config.xml restricts MBean details from getting accessed from remote machines without authentication.
Set the attribute value as false.
For more information, visit http://e-docs.bea.com/wls/docs81/config_xml/SecurityConfiguration.html#AnonymousAdminLookupEnabled
In the trial version of WebLogic, connections are restricted to a maximum of 5. So the request from Applications Manager might be blocked.
Try restarting your WebLogic instance and ensure that Applications Manager collects data before the limit of 5 connections is reached.
The proxy server is blocking the connection to the WebLogic server. The proxy server might have been configured to allow only HTTP requests.
Contact your system administrator.