I have this same problem and I do not have an underscore in my machine name.  Does anyone else know what the problem is or could be for this?  I can login fine to chrome / firefox / safari ..etc...BUT not IE.  I am not sure what could be wrong, but something is.
My site is hosted on my machine and my ISP does not allow static IP and blocks port 80 so I had to set up port forwarding through no-ip to point to my dynamic IP on my machine, so my alias is my dynamic ip with the forwarded port on which my site is running on. (99.99.99.99:8080)  Could this be a problem with IE since the previous example uses an underscore?  Any comments would be great.  Thanks in advance!

I'm sorry, I should have been more descriptive in the dynamic portion on the post.  No-ip.com handles dynamic ip's as well as port forwarding.  Everything works with the "dynamic" portion of the ip address, whereas it gets updated through a No-ip app on startup. I still am clueless as to how login works on every browser but IE.  If you have any ideas I would greatly appreciate it.  

You said port forwarding has known issues?  Do you think this falls under "known issues"?

Thanks,
CB

I found the issue with IE.  It doesn't login if I mask the url of the port redirect + no-ip handling of dynamic ip address.  Say my ip address is 74.56.33.95 and the port I am using is 8080 - my url would be 74.56.33.95:8080.  Instead of showing that, I masked the url to show "mywebsite.com" and this works great in everything but IE.

If I unmask the url (show the ipaddress with the port) then I can login just fine with IE but if it is masked, then I cannot login.  

Is there an easy way to show the url I want (mywebsite.com) instead of the long ip address?  I have tried an alias with "mywebsite.com" and that just puts it in an infinite redirect loop as it goes to the no-ip.com port redirected path and then hits dnn alias of "mywebsite.com" then goes back again to no-ip.com redirected path.  I am stumped!