Pinnacle 23. Windows 10. Never had problem before. Been using for 10+ years. 

New project. Trying to import files (MP4, MTS, MP3, JPG) via both Quick Import and Drag & Drop methods. Lower right corner, progress bar appears, maybe even a flicker of a file name or two, then it goes away. No files made it to the project. No error messages or incompatible messages whatsoever. To be safe, I JUST upgraded from Windows 8.1 to 10, and also reinstalled Pinnacle 23. ANY clue as to where to BEGIN to look for the cause here?  

In PS23 you can't import directly into a project. I think you mean a Project Bin? I assume you are adding to an open bin? Has anything appeared in the Latest Imports collection?

Where are stored these files on your system ?

I have been doing Quick Import into the Project Bin, as well as opening up the file folder and dropping the file directly into it. ALSO, i have managed to have a FEW files transfer, MTS, but not all. They were all taken with the same camera, same time, and are all MTS files. But I now cannot drop MP3 files either. I can drag and drop, and it generates the progress bar at the lower right corner for a second or two, but it's like vanishing into thin air. 

 and... i have been gathering files from different drives. The mere fact that it allows me to open up the file folder, even if I was dumping directly from an SD card, means it's seeing SOMETHING I can drag and drop. This is all of a sudden as well. 

 I JUST upgraded to Win 10 the other night, reinstalled PS23 twice, and it made no difference. I just cannot seem to find the common denominator. 

You may try to hard-reset PS23 :

Rename C:\Users\Yourusername\AppData\Local\Pinnacle_Studio_23\Studio\23.0 to C:\Users\Yourusername\AppData\Local\Pinnacle_Studio_23\Studio\23.0.old

Launch Studio, try to import.

One big side effect => your library has been reset.

 

If the workaround failed, delete the newly created C:\Users\Yourusername\AppData\Local\Pinnacle_Studio_23\Studio\23.0 and rename C:\Users\Yourusername\AppData\Local\Pinnacle_Studio_23\Studio\23.0.old to C:\Users\Yourusername\AppData\Local\Pinnacle_Studio_23\Studio\23.0

Same problem. Started an online chat with Corel, but not able to give an error message ID, so that avenue is worthless.

Same problem, same issues. Tried hard-reset workaround which solved problem for one project. Started new project and issue re-appears with attempting to import another mp4 video.

what is the  source (what created it) of the mp4 video.

How are you trying to import? There are a few ways. 

Did you try uninstall/reinstall with registry clean along with deleting the hidden appdata folders?