The fix for this issue is now available in the latest update 14.0.2 of InDesign 2019.
If you are unable to see the update notification yet, open the menu (three dots) from the top-right of Creative Cloud desktop application and click on ‘Check For App Updates’.
Once the update button is shown against InDesign, click on the button to update to Id 14.0.2.
If you face any problems with the new update, do let us know in the comments.
I posted first about this in the Community forum. The problem still persists. I have since upgraded to Mojave, and am getting the same error message in IC 2019 whenever I attempt to open an .icap file.
IC 2018 still works.
And whoever the dude is who keeps arguing with me and others, telling us that we should just not use this workflow and use Dropbox instead, don't bother saying it again. The software should work as advertised.