Hope you are doing well and sorry for the trouble. As described, document are returned black with white blocks where you have redacted.
Is this a behavior with a particular PDF file or with all the PDFs? Please try with a different PDF file and check.
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 21.05.20048 installed. Go to Help > Check for Updates and reboot the computer once.