Repeated reboots while saving an activity

I upgraded to 2.70 and since then my two hiking activities are rebooting the watch during the activity saving. After the watch restarts the activity is loaded back. I save it and it reboots again. That repeats multiple times until it finally succeeds (5-10 attempts). Probably related to elevation but it is just a guess. My two running activities saved fine on first attempt.

Any idea what is going on? Bug?