A few housekeeping notes:
1. Due to spammers we had to disable the ability to attach files for portions of each day which is why you were unable to upload an image. Things are back to normal now and you should be able to upload without issue. our apologies for the inconvenience.
2. I've moved this post to our Server Administration space where other admins can take a look.
3. If you ever have issues with the operation of the Community itself, please don't hesitate to tag our administrators or email us at firstname.lastname@example.org.
Now on to your question! The first thing I would verify is that the Server url is correct:
Then, I would try opening the exact subscription link in various browsers and let us know what happens.
I was going to suggest the same link as Diego. One other item to have a look is what kind of authentication are you using? We are using Active Directory integrated with auto-logon and it works well for us. And we don't run internally with SSL.