Show us the import dialog box when this happens.
There is no dialog box appear.
It just happen like that (shown in the images) the loading bar hangering there forever, not loading and no error dialog box pop-up.
I can still interact with Lr like normal, but the photos just would load into the Lr.
I try add, move, and copy. They didn't work in this case.
You have to get an Import dialog box. File->Import Photos and Videos...