What it basically sounds like is that the built-in web server is accessible - it's getting the stuff off Port 80 - but that seems to be what loads the java applet, which then talks to the M1 via the secure port, which by default is 2601. If you haven't changed that, test connectivity to that - preferably from the outside, but attempting a telnet - so, from a command prompt, you can try typing "telnet mym1.dyndns.org 2601" (or whatevever dns name you chose).
It probably won't tell you anything useful, but if it connects, you'll get a blank window you can type in (it won't echo anything back). If that works, then it's likely with your browser or PC.
Is this a standard PC, Windows XP, etc? I know that my Mac on Safari doesn't seem to connect, but I've put about 20 seconds into troubleshooting - so I don't know if that's fixable. PC's with standard security IE seem to work just fine.