I'm using a 6+ as well but it started before I switched from my 5s. It's pretty frustrating especially when I can't tell my dad what he needs to do to fix it.
I have uninstalled and reinstalled the app (multiple times), logged off and logged back on using my and my dad's accounts, rebooted the phone, tried on my Android tablet. There have been a couple times that it has displayed the current readings after I've tried one of these but then it just starts returning the error the next time I go back to it. I know that it's connecting to the accounts because in the system settings it is displaying the sensors' names.