I had this problem, too, (or at least a similar one, as there are apparent many reasons for this error message) over and over as I imported photos. The message is accompanied with this icon as it prepares to import:
The solution, which makes the process MUCH more time consuming, is to SHUT DOWN Photos and turn it back on. Then choose a much SMALLER batch (sometimes as few as 5-25 but usually about 100, which for me, was a real pain because I had to import 30K photos). Then WAIT until ALL the teeny thumbnails show up while it's preparing for import. If you just the gun, they the icons above will show in their place. NOTE: If you are importing only a few, the downloading of the teeny thumbnails will happen so fast, you won't have to wait to see them populate the screen.
I learned these tricks on my own after spending weeks trying to get all my photos uploaded into a new Photo Library. Apple does not provide an error message that correctly states that the error is trying to upload too many photos at a time, or you're trying to rush it, attempting to import before the program is ready to do so. It just gives the scary "unrecognizable" error message instead, throwing this problem in with a whole host of other problems that accompany this message, making it much more difficult for users to pinpoint the problem.
