I had this problem for the past few days, plus half the time Photos wouldn't even recognise that there were new photos to import. When it did, many preview images were just grey boxes and even the ones that loaded preview eventually timed out and gave error -9934 on importing. What eventually worked for me:
- Connected phone to a family member's Windows10 PC and access the new photos via both folder copy-paste and by importing from within Win10's Photos app
- After connecting phone back to my Mac laptop, it now worked perfectly, imported 1600 photos in a matter of minutes.
I have NO IDEA why or how this worked. But since it did, it's probably worth trying.
Apple Support's other suggestions were using the Recovery utility to reinstall the OS (they say there's an option that won't wipe your data / reimage the machine, only restore system files to their standard in case some are corrupt, missing, etc.). But I wouldn't definitely back up your machine before trying that.