Photos not showing thumbnails in iPhone import view

If we select Devices (Geräte) => myname Iphone it loads some thumbnails but then stops and only displays gray squares.

What we tried first:

• Reinstalled macOS
• Reinstalled iPhone (Deleted completely and then restored)

But it did not solve the issue.

What helps for one or two Photos application starts is to rebuild the library (holding the option and command key while clicking to launch the photos app in macOS).

This is a terribly annoying workaround because we have to do this very often and it's time consuming. Sometimes it doesn't work and we have to try multiple times.

How can we fix this?