Why would you think this is totally a LR problem?
To me it sounds more like a OS X problem since you've only had problems since the OS upgrade. Why not restore your system from the Time Machine backup you made just before you installed the new version of OS X.
Thanks for your reply. In fact I do not think this is totally a LR problem, but I am trying to find a solution without reversing the Sierra installation.
But maybe this is the only option left.
Try launching Image Capture and if your camera is recognized as a divice download the images to a folder in Pictures, Then use:
Import >> Add
Have you tried using a card reader to copy the files to your hard drive?
This is generally safer and faster, especially if the card reader is built into your computer.
Fewer pieces of hardware means a smaller risk of anything going wrong.