Just leaving a note here, Reality Converter is nowhere to be found on the suggested https://developer.apple.com/download/ link. Not Applications nor in 'More' tabs either.

Having the same issue here. For sure it wasn't happening last week. I did update Xcode to the latest one. What you can do is go to your keychain, look for the bunch of letters regarding your certificate (on the search bar) and tinker with it (set to always trusted then back to system default). Then try to build again.