I don't know if this will be any help but I'm in exactly the same position with the school I work at. We have an LEA ISA server and a TMG server sitting between the outside world and our moodle site, in school the site works just fine (certificate issues notwithstanding), but not outside.

I've looked up this error and one forum reports it could be a time related issue. I know from my viewpoint that the internet time and the time on our server are about 3 minutes out, the server is part of a domain so it needs to have the same time as the PDC; which is about 3 minutes slow.

The problem also appears to only affect Google Chrome as IE 9 doesn't show the same problem, it just won't display the https login page (having just tried a remote login to my home computer). Change the http to https and everything appears to work apart from a constant message about secure and unsecure content (so in chrome stuff like the dock disappears, or in school no theme is applied). I found a line in the config.php file inside moodle $CFG->wwwroot=... where if you change the http:// to https:// everything works, but the drawback is that forces the whole site onto https which can cause content problems as the browser can no longer cache material, on a big site it can cause a really significant slowdown.

Any help with this would be greatly appreciated as I can't see a solution apart from https the whole site.