Photos blurry after editing in Photos for Mac and no thumbnail

When I edit a photo in Photos for Mac and then back out to the view of all photos, no thumbnail appears anymore (grey tile in the case of dark mode). Also when I then want to view that photo in full screen, it appears either blurry or completely white. When I click on edit again, the photo appears sharp (as it should be). In the activity monitor, a process is running 'com.apple.photos.ImageConversionService' that uses >100% of my % CPU. This slows down all other processes on my Mac.

Specifications of my system:

MacBook Pro (13 inch, 2016, Four thunderbolt 3 ports)

Processor: 2.9 GHz Dual-Core Intel Core i5

Memory: 8 GB 2133 MHz LPDDR3

Graphics card: Intel Iris Graphics 550 1536 MB

I am running macOS Catalina version 10.15.5 (up-to-date) and Photos version 5.0 (151.19.150).

Things I have tried already:

• Removing all changes (rotation, cropping, light adjustments), now a thumbnail is created and I can view the image in full resolution in full screen mode. However when I make an edit again the issue reappears.
• Repairing the photos library (holding down option and command while opening), did not fix issue
• Making a new test photos library, uploading some new photos and editing them, issue remained
• Restarting the system, issue remained
• Booting up in Safe mode (holding shift while booting up), issue remained.
• I have found that when I only edit one photo in a test library, the 'com.apple.photos.ImageConversionService' process finishes after about 15 minutes, after which the image has a thumbnail and can be viewed clearly in full screen mode. However this time is unpractically long when you have hundreds of photos per week to edit, and usually the loading time for a full resolution image only took a fraction of a second.

I hope somebody can help me!