I had the same issue, and after checking more detailed Xcode logs (here is some info on how to do it: https://stackoverflow.com/questions/27536271/how-to-collect-xcode-log )
I did found out that one of the internal folder of my app had the same name as an integrated folder of one of the SPM Package and for an obscured reason, this triggered the error.
If you either exclude the folder from the package (if not needed and you have control over the SPM package you use) or change the name inside the app (if you can), it makes the error disappear.
nicolas_s
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