Please try and reset the Lightroom Preferences back to default.
Location of Preference file:
C:\Users\[user name]\AppData\Roaming\Adobe\Lightroom\Preferences\Lightroom 6 Preferences.agprefs
Rename the Preferences file to Lightroom 6 Preferences.agprefs.old
Launch Lightroom and use it.
Let us know if that helps.
I tried this twice, and it did not fix the problem.
I tried to do the same with "Lightroom 6 Startup Preferences.agprefs", but when I started Lightroom, it told me it had to convert my catalog. So I exited and restored that config file.
In Hang during import - Lightroom Forums, I found:
Hi, I have encountered the same issue with LR 6.3 standalone and it got fixed when I connected To Adobe Creative Cloud before launching Lightroom.
I started the Creative Cloud application, and I wasn't signed in. I signed in, and the import succeeded.
So I think that was the problem: I wasn't signed into the Create Cloud, at least as far as the Cloud app was concerned.