I think I can help with this thread. This has been a frustrating issue for me, since I feel the photos app hasn't been the easiest to work with since transitioning from iPhoto. I contacted Apple support to shed some light on this and the support member did say this is a common issue, but also seemed as if they have several causes for this issue, depending on the individual user. I made the call while i was at work, so the agent was only able to give me suggestions, as i was not in front of my iMac. He asked if i had a dropbox account. I wasn't sure, so when i returned to the iMac, I searched for that application, but i did not have a dropbox account. This lead me to believe that I might have possibly downloaded a program that allows access to my photos. I downloaded the amazon photo app a few months back ( this amazon photo app was downloaded after my last successful photo backup). I deleted the app from my iPhone and plugged in my iPhone, only for the dreaded "white screen" to appear in the "import" tab with no photos to preview. I followed the procedure listed from user "Old Toad" and "RobertF8" from this same thread. After performing the soft reset and unlocking my iPhone, the "no photos" message appeared on the Import tab. I disconnected my iPhone and reconnected it right away. This brought up the imports tab again, recognizing my iPhone and sure enough, all of my photos appeared on my preview list and the "Import all" was finally clickable. This has been an ongoing issue for me for several months, as it recognized I had about 5,134 photos to back up from my last back up.
I think this might vary from user to user, depending on any non apple photo app that allows access to your photos. I feel this was a lousy way to go through these steps, as apple does not have a solid fix to this issue yet. If you have dropbox, I suggest closing the app from your iMac as well. Good luck.