Please try to create a new catalog and check importing in that.
It might be the catalog issue.
I tried to create new catalog, where I was able to import photos from folder in computer (downloaded to folder in computer from card) - in old catalog i tried to import from folder through "synchronize folder" (new folder is in folder with photos that are already in catalog), it shows me cca 300 photos for import, but after few minutes of "synchronizing folder" it said that "no photos to import" or smething like this...
From another card, when trying to import directly from card, it shows same problem like before - "no photos found" (new catalog)
Also I tried to import photos to new catalog from old LR5 catalog, and it says that I must upgrade old catalog for LR6. After few minutes of upgrading it says that Lichtroom "can not import from this catalog at this time. Unexpected error when creating temporary copy of the catalog."
Just confirmed that with Lightroom it is a known issue of sync that is going on with Lightroom's latest version.
So if you want you can revert back to the previous version of Lightroom. : https://helpx.adobe.com/lightroom/kb/roll-back-to-prior-update.html