Apparent Dysfunction with DC Fill & Sign

Good day.

Adobe Acrobat Reader DC 2019.021.20047 on Windows 10.

The Fill & Sign feature was working as desired until yesterday. Not aware of anything I've done to change configuration, except one possibility I'll mention after describing symptoms. What's happening now:

1. Open Document. Any document.
2. Select Fill & Sign.
3. Prompted with "Who needs to fill and sign?" Select "You" (i.e. me)
4. Document displays with normal-looking Fill & Sign toolbar at top. Mouse cursor displayed over document is the text entry tool (I-beam with "AB"), as expected.
   Here's where the fun starts. Either:
5. Click on the document. Cursor converts to I-beam. Anything typed does not display. -or-
   Attempt to drag my signature image into the document from the toolbar. When I attempt to select my signature from the Fill & Sign toolbar > Sign drop-down, the drop down menu disappears. Cursor goes to open-hand when I hover over my signature and pointing-hand when I attempt to select the signature. There is no indication on the document that I've dragged anything, and nothing displays in the document when I release the LMB.
6. Document looks at this point like nothing has been filled or signed. Save document, close.
7. Reopen document. Filled text and signature are displayed--but, of course, not correclty or in the desired place, because I had no on-screen feedback as to what/where I was typing/drag-and-dropping.

I've tried repairing DC from within the application, from the Windows Apps and Features settings. I've also uninstalled/reinstalled DC. No change in behavior. I've tested this behavior with the Adobe-provided Fill & Sign demo tutorial document and I'm getting the same behavior.

The afore-mentioned one thing I have done that could have somehow changed the configuration is that in the last day I have  opened PDF documentd that were generated in-house using Apache FOP from DocBook XML using XMLmind XML editor. Not sure how that could possibly relate to this, but it is the one thing I can think of that's changed.

TIA for your help.