What is the version of the Adobe Acrobat DC you are using? To check the version go to Help > About Acrobat and make sure you have the latest version installed Go to Help > Check for Updates and reboot the computer.
Also, please try to logout of the application, reboot the computer once and relogin to the application and try to sign and check if that helps.
You may also try to connect the machine to a different network and see if that works for you.
Hope you are doing well and thank you for trying out the suggestions. As mentioned its an issue with the PDFs exported from your workpplace system. Would you creating the PDF using Adobe Acrobat pro DC application and see if that helps.