Long time Adobe Acrobat DC user. Grey screen when signing.

As of a few days ago I'm unable to sign any PDFs in the normal manner. I normally use 'Digitally Sign' as follows. Highlight where the signature field goes and wait for the dialogue box to appear for the password. Except it doesn't appear and the Acrobat window greys out (I can just see the signature marquee, so it's semi-transparent, fwiw I uploaded this) and key inputs are bonged. The only way out is to 'Esc'. I've rebooted PC (running Win10). Checked for updates - it's up-to-date. Manually repaired Acrobat. Searching the community doesn't reveal anything that looks useful. Or IT guru can't help although notes that he's had several users with strange, but not identical issues. I tried 'Fill And Sign', Request Signatures. The resulting dialogue box appears but is empty and white. Stamp Tool works.