We are sorry for the trouble. As described you are unable to add the signature to the PDF file as the screen changes to grey.
Would you mind sharing a screenshot of the issue for a better understanding?
Make sure you have the latest version of Acrobat. Go to Help > Check for Updates.
If it still doesn't work, please try to repair the installation (Windows Only) and reboot the application, and check. Go to Help > Repair Installation.
If it still doesn't work, please try to turn off the protected mode for testing (Windows Only). Go to Edit > Preferences > Enhanced Security > Uncheck 'Enable protected mode at startup' Turn off the protected mode and uncheck enhanced security > Click OK and reboot the computer. Note: Turning off the protected mod may possess security risks, please turn on the security after testing.