This article describes why you can get "Error response Error code 500. Message: Internal Server Error" when accessing any secure (https) site.
This error can happen when you use web acceleration software such as AT&T Worldnet Accelerator. While non-secure websites work with this software, secure websites do not. To solve the problem, you should configure Firefox to not use the web accelerator for https.
Click the menu button
and choose Options.Preferences.
Select the Advanced settings panel.
Click the Network tab.
If Manual proxy configuration is selected, continue to the next step. If not, these instructions won't solve your problem.
Deselect Use this proxy server for all protocols.
If it's not selected, leave it as is.
Delete any text in the text box beside SSL Proxy:.
To the right of SSL Proxy:, set the Port: to 0.
Close the about:preferences page. Any changes you've made will automatically be saved.