Cannot import a PPT file -- no way, no how!

Hi,

I'm in the first stages of a PowerPoint to Storyline conversion project for a client, and trying to something I've done without trouble many times before--import a PowerPoint into Storyline using the Import PowerPoint dialog.

Every attempt results in a "Failed import PowerPoint presentation" message.

Here are some things I've tried:

• Confirming/completing each item on the list given at the page linked in the error message:  https://articulate.com/support/article/Articulate-Storyline-360-Failed-to-Import-PowerPoint-Presentation, including "repairing" my Microsoft Office installation.
• Uninstalling all my Storyline apps; reinstalling Storyline 3.
• Asking a remote colleague to try importing the original PPT using his SL360 installation. (He got the same error message.)
• Importing several different PPT files, each from different eras and versions of PowerPoint.
• Importing different PPTs into Storyline 2, Storyline 3, and Storyline 360.
• Rolling back to the early May 2019 version of Storyline 360 from the latest update.
• Opening a blank Storyline file and importing into that. Starting from the opening screen and using the Import dialog. (Neither workflow works.)
• Moving the PPT from a folder to my desktop, and restarting the whole computer.

In short, I've tried just about everything I can think of and unfortunately, I still can't import a PPT to Storyline.

I hope that Articulate can offer some insight here!