Update: curiously, I don’t get the error when I do a release build (cordova command: run ios --release) but I still get it when doing a debug build (cordova command: run ios --debug).
Still pretty urgently need that downgrade to 8.4.2.
You’ll have the same problem with 8.4.2. Your certificate for debug builds in invalid. This is flagged by XCode, which VoltBuilder uses for iOS builds.
You will need to create a new certificate and mobileprovision file.