My iOS-App worked like a charm with Firebase 8.15.0.
Now I changed the Firebase package dependencies to version "9.0.0 - Next major" and get a lot of errors, which I can't link to the breaking changes related to this major release.
I already cleaned the build folder, restarted Xcode and my MacBook, but nothing helped.
Most likely the underlying problem has to do with the warning "Could not read serialized diagnostics file: error("Invalid diagnostics signature")":