Also, make sure you have the latest version 8 installed. If you have got the PDF as an email attachment, try to download the PDF to your computer locally and then try to sign the PDF using fill and sign tool. Also, you may try with a different web browser and check. Have you got the PDF file via Adobe Sign? If yes, please try to clear the cache of the web browser and try again. Would you mind sharing the screen recording or a screeshot of the issue for a better understanding?
As described you are unable to add the signature to the PDF file as the screen goes dark.