A 400 error is an error code that means that the ping to the server timed out after waiting a long time without a response.
Here are some helpful tips that may help you with this error:
Make sure you are not attempting to use a POP account with TouchDown. If the address you have entered into Quick Configuration is for a Webmail client, then TouchDown will not be able to connect.
Make sure that in TouchDown Advanced settings you have not changed the ActiveSync device string to anything other than "TouchDown".
Make sure your server does not have an untrusted security certificate that may be giving you the boot without reporting an "untrusted certificate" error.
When all else fails, try rebooting the device. Some devices have a tendency to "lose" their IMEI number to report to the server, and therefore will go unanswered. However in most cases rebooting the device (and often times a battery pull may be needed as well) will restore the IMEI number and subsequently the 400 error