I’m not sure if your hardware is compatible with older firmwares, but I recall about a year or more ago when I upgraded firmware and ran into this problem too with plane. I didn’t have time to roll back firmware or bench test, but could you try copter 3.6.9 or 4.0.0? There are definitely a lot of parameter changes since then, so be sure to back up what you have now. Also see Internal error 0x2000000 - #13 by Steven_Young for more similar discussion. Maybe before downgrading, try a few more tests.