Are you trying to use Fill & Sign feature via Document Cloud web interface or working on desktop application Adobe Reader DC?
I assume, the error comes up when you are sending a specific document for signature, did you try signing another one? If you are at work, then there must be some restrictions over the network. You may try accessing the URL using different web browser, Adobe Document Cloud and if that doesn't work then get in touch with your IT team to white-list the path.
Hope this works,
I'm having the same issues at home on my chrome laptop. It was working a few weeks ago. Nothing on my network has changed.