I've been using RaceChrono for a while and it was running reliably. However I've run into the same two problems on my last two track days. Around the third session of the day the app locked up mid session. Then on my last session of the day the app never detected the start of the session. Is there an error log that I can look at to see why it's having problems?
I'm using version 7.6.3 with an OBDLink MX+ and Dual XGPS150A.
I assume the hang and the fail to detect is separate issues.
For the hang issue I would reboot the phone first. If the hand doesn't happen again, I would just deem it one off incident. If the hang happens regularly, I'd try to isolate the cause. Did you record video with the built-in camera? Then try without it. Did you open the map screen when recording, then try without it.
About the auto-detect issue. Do you see the lap times of the session, if you manually select the track for it?