Posted in July, and filed Feedback about it with Apple. After some back and forth they said **"Thank you so much for providing the built products! We’ve identified that object files for this project are built as universal. There’s a known issue with the new Previews execution mode that projects built in debug mode as universal can run into this exact error. We’re working on a fix. But in the meantime can you check your scheme build settings? Open up the scheme editor by choosing the “Product > Scheme > Edit Scheme” menu item. Click on the “Build” sidebar tap in the modal window. You should see a control in the middle labeled Override Architectures. We’re pretty sure you have yours set to Universal. If you set that to “Match Run Destination” then your debug build products will just focus on building the architecture that you’re targeting to build-and-run. That should be sufficient! Please let us know if now. Note that this only affects local development. If you archive or submit to the app store all the architectures you are targeting will be built as usual."** This has certainly made it more stable for me…

No, still not able to resolve this. Yet to try Xcode 16 beta though…

Seeing similar. SPM previews work—when selecting appropriate scheme—but main app previews don't, failing to launch with very unhelpful message. Tried all the usual resets, cache deletions, restarts and rebuilds, to no avail. Have filed FB13954069 with report and diagnostics.