Certificate is valid. I've tried on an iPhone6 and 6s here in the US with no issue. I have seen remotely by video someone else having the issue with the 6s and plus. The problem has happened on a few iPhone 6's in Europe so I cant get direct access to them. If I had one locally here with the issue in the US, I would connect it to B4i to see what the issue is :-(. I have not seen this problem on the 7, 8 or X. All iPads are working. I have checked on the server side and the phone never hits the server, so I don't think its getting out of the app. Very strange!
As an aside, B4i is great! So easy to use and powerful.