After upgrading from 3.3.24 to 4.0, the main web server seems to be fine, but the admin site isn't working, get a 503 error. Here is the error.log entries:
2009-03-20 11:26:42.095 [INFO] [67.197.xx.xx:50989-0#_AdminVHost] ExtConn timed out while processing.
2009-03-20 11:26:42.095 [INFO]...