What was the issue exactly and how did you solved it ? Because I am having the same problem except that trying yoursite.com/index.php/admin only cause firefox to tell me that I have redirection issues.
Edit: Additionally, a colleague created a panel called secure in which we can access the CMS but log to a custom panel of the CMS and only this panel seems to be working, can you advise please ?
Edity #2: I have found my mistake :
it was my own mistake, my misunderstanding of the new userform and carelessness.
I had put an email sender, without the email recipient. That was causing the server internal error.
However, I think that it could be great if the form should checked if there is an email recipient and send an error message and not simply die if it doesn't...