Same problem here. iPhone 6, which imported fine with LR 5, behaves as described above by Darrall. 2015-07-09
I'm STILL having this very serious problem! LR absolutely refuses to import ANY photos or videos from my iPhone 6+. Every photo on the grid is greyed out "PREVIEW UNAVAILABLE FOR THIS FILE", regardless of the destination I set. I do have ~2500 photos on my iPhone but why would this be a problem?
When I select even one photo and try to import it, LR hangs and cannot be quit. I have to force quit it.
This is obviously a serious bug, and no-one seems to have a solution!
How am I supposed to get new photos and videos into LR?
This is a show-stopper which makes me begin to regret moving from Aperture which at least did work reliably.
Having the same problem. Need a solution. Hope one comes quickly.
Are the photos in the Camera Roll?
Can you import some of them with Image Capture? that at least might isolate the problem to Lr.
Interesting. I imported them into Photos on the Mac and they were there.
Went out, ran errands, came back, decided to try LR import again, plugged the phone back in and all previews and images showed up and were able to import directly.
I know that they can be imported into Photos, then exported to a desktop folder, but that seems so non-straightforward and a PITA.
Same problem... Lightroom CC 2015 and iPhone 6. Image Capture works fine, but Lightroom hangs when importing. If I cancel, the import functions are then disabled on the File menu and Lightroom needs to be restarted for them to be enabled again.
I have around 10 GB of photos on the iPhone and a few videos, but Lightroom correctly identifies a delta of only around 240 photos since the last time I managed importing them (maybe in a previous version of Lightroom?--can't remember when that was vs when I last upgraded Lightroom CC).
I know there are work-arounds to get my photos, like Image Capture, but this is annoying from an application like Lightroom... Plus, I'd like to run my presets on import, including file renaming, although for the moment I'll probably write some scripts to get around this...
I had the same problem with my iPhone 6 and LR6 - and was able to solve the problem. What worked for me was to *never* let the phone enter standby mode while synchronizing. Now everytime i'm synchronizing i set Settings->General->Auto-Lock to Never before i connect the phone.
I have this problem as well: iphone 5 v8,4, Lightroom version: CC 2015.1.1, Operating system: Mac OS 10 Version: 10.10 
Image capture sees photos so it is not a connection issue, it's an intermittent problem for me but extremely annoying.
I have the same issue. I'm running the Lightroom CC with the November 2015 update (return to normal import format). I tried what sereij recommended. It worked a few months ago but not today. The phone keeps disappearing from the source list before i can get the folder location set up.
I tried this. LR does not see the preview file and will not import and gives the message that the file is not imported because it could not be read. It is a jpeg file.
this is not new, actually. This bug creeped into LR long time ago (I've had it at lease with 2 or 3 iphone generations back, and now I see it with 6S Plus 128Gb), and it is surprising that Adobe has done nothing as yet to address it. As of January 30, 2016 this bug is comfortably there, and easy to replicate: get and iPhone with 64 or 128Gb of memory, and load it with several thousand pictures. On the first attempt at import LR will almost always show you empty thumbnails with "unable to show preview" on them. If you persist, reload LR a couple of times, keeping the iphone connected, you might get lucky and see the previews. This is unacceptable, please fix this, Adobe!
I agee with sunlit777 - this is not acceptable - to have Lightroom 'not be able to read' iPhone image files is just ridiculous
I do have an answer to the problem where you can't show previews - if that's the only problem you have - and that is to turn on off your wifi connection while downloading from the phone.
Unfortunately I have two other problems. One the iPhone keeps disconnecting if I try to download from ImageCapture - even though no such problem when connected to iTunes.
The other is the aforementioned inability to import from the phone as Lightroom says it can't read the files, even though it shows previews and the files are jpegs.
As a temp solution - the only way I have been able to import to Lightroom (short of emailing photos to myself) is to download from iCloud backup. To download a whole day's worth of photos, you click on 'moment' on the right above that day's photo, and all the photos will be selected. Once downloaded and imported to Lightroom, I go back and delete from the downloads folder.
Adobe please fix this. I used to be able to just drag from the photos app straight into Lightroom - but now this entire import process it just painful.
I've had this same problem for a long time now and it's ridiculous! Come on Adobe!