Upgraded from 2.7 to 2.8.4.
I can access and edit any page in the admin area that I've tried, except that the dashboard at /wp-admin/index.php give the error:
Safari can’t open the page “http://mydomain.com/wp-admin/index.php” because the server unexpectedly dropped the connection. This sometimes occurs when the server is busy. Wait for a few minutes, and then try again.
In Chrome I get:
This webpage is not available.
The webpage at http://mydomain.com/wp-admin/ might be temporarily down or it may have moved permanently to a new web address.
More information on this error
Below is the original error message
Error 324 (net::ERR_EMPTY_RESPONSE): Unknown error.
In Firefox I get a blank page with a blank show-source page.