I was searching the internet for an answer to the same question. In my Enterprise account, I go to certificates. I now see "iOS Distribution" cert and "iOS Distribution Managed" cert. Two different creators, two different expiration dates.

I ran into this exact same issue. The file was in my bundle, I could see this when running my iOS app on my simulator and using Finder to inspect the app bundle. If I switched String(contentsOfFile: String) to String(contentsOf: URL) it worked. -Ed