We are sorry for the delay in response to your query.
Are you using the Adobe Reader DC desktop application to use Fill & Sign feature? Or using DC web interface Adobe Document Cloud ?
Looking at the error message, it seems to be a network connection error.
1- Have you checked with a different pdf? If not then try uploading a different image on other pdfs.
2- For Adobe Reader DC desktop application check the latest update. Open Reader>Help>Check for update.
3- If using any antivirus, then you may turn it off for a while and check back.
[Antivirus software can help protect your computer against viruses and other security threats. In most cases, you shouldn't disable your antivirus software. If you have to temporarily disable it to install or activate other software, you should re-enable it as soon as you're done.]