This error will happen when phpList is unable to fetch the URL that you have entered for sending in the Content section when creating a campaign.
This does not necessarily mean the URL is incorrect. It only means that phpList is unable to fetch it. The error is between the server where phpList is installed and the URL that you entered. If you can load the URL, then it will be an error somewhere else.
Double check the spelling of the URL you have entered. If you think the URL is correct, and you still get this error, please contact your network administrator.