I have a 510 and I'm trying to sync it to an iPhone with no luck. I'm having this issue for months now. It used to work really well last year, bug this season (2015), rarely works.
I was hoping it was my phone (5s), which I was running out of memory , but I got a new iPhone 6 plus and I'm still having the same issue, 510 connects, I get the red error message at the bottom of the screen and although the app shows the phone as connected, live track and upload do not work.
iOS 8.4.1
Garmin connect build 2.13.2
510 firmware 4.2
Am I the only one STILL having this issue?