I had a very strange crash with my quad. I made dozens of successful missions with APM 2.0 - 2.9.1b Arducopter firmware. I felt this platform was very stable and did not upgrade to 3.0.1 on this frame.
Well I had a crash few seconds after taking off; the propellers just stopped! I realized that for some reason the APM was not powered anymore from ESCs. The J1 jumper was soldered long time ago and even after crash It was ok. now The board can be powered only by usb cable; it is not possible to feed the board by ESCs as usual... I'm afraid it is an hardware problem, but I cannot explain how it happened and what is wrong now. Have you ever had such problem before?
Thanks for your help.