If I had to guess I'd say it has something to do with allowed security protocols/versions supported or enabled in your browser. Old school System Manager(Pre CDOT) is pretty finicky and was missing a lot of features that are only accessible from the command line. You're probably better off using the CLI. Try connecting with a terminal client like Putty. If you can't connect with SSH try telnet which might still be enabled on a FAS2020 system.