Hope you are doing well and sorry for the trouble. As described, save as prompt is not working it just shows a white screen and sits and spins unable to save the doc.
Is this a behavior with a particular PDF file or with all the PDFs? Please try with a different PDF file and check.
Also please try the following preference settings and see if that works for you. Go to Adobe Acrobat > Preferences > General > Uncheck the options 'Show online storage when saving the file' and 'Show online storage when opening file' > Click OK and reboot the computer
Make sure you have the latest version 20.12.20048 of Acrobat DC. Go to Help > Check for Updates.