Hey wouter_10,
Let's take a step back. You mention needing to remove the photo's from your iPhone to your Mac because your iCloud storage is full. Do you not use iCloud Photos on your Mac too? If so, removing the photos from your iPhone won't help the iCloud storage levels. Check out the following guide for some great information and steps for managing your iCloud storage: Manage your iCloud storage - Apple Support
We also thought you'd find this resource helpful to know the difference in device and iCloud storage: What's the difference between device storage and iCloud storage? - Apple Support
If you still need to import those photos to your Mac, start with ensuring your Mac and iPhone are fully updated. Updates help by installing bug fixes and include security updates. If an update is needed, back up first:
Update your iPhone or iPad - Apple Support
Update macOS on Mac - Apple Support
If the behavior continues, try importing the photos using Image Capture instead. Here's a resource to help with those steps. You don't mention which macOS is installed on your Mac, so be sure to choose the correct one under "Select version" at the top of the page: Transfer images in Image Capture on Mac - Apple Support
Let us know how it goes.
Take care!