Just to save anyone some headache, in case you forgot your doman server’s settings username and password:
Shut off your Sandbox
Navigate to the following directory:
Open config.json with notepad or an equivalent text editor
Remove the http_username and http_password keys from the json. Save.
Restart your Sandbox!
Note: This is for Windows users. I don’t know where config.json lives on OSX, but the settings for your server should be in that file.
This will erase your server settings username and password.