I was having this error today and restarting Xcode did not fix it but closing out all the tabs within Xcode fixed the problem for me.

I think the issue isn't that all package imports break Xcode Previews. I am currently building a Swift Package where I use SwiftUI previews. I am integrating 2 packages simultaneously into this target and integrating the first one was a breeze. As soon as I introduced the second one, my preview crashes and I am getting an error: I think the issue here is that some trait of that second Package causes Previews to break. For this second package, there are some setup steps that I cannot take in a Swift Package like Info.plist entries. It would be great if Xcode or the Xcode preview failure error would give an exact reason why this second package was unable to be loaded.