Same happened to my Pico-z. I followed the advise above but the problem did not disappear.

By looking closely I noticed that the tail motor vibrated (left-right). This caused it to exprience too much friction above a certain speed of the main rotor, and this friction triggered the safety mechanism for stopping all motors during a crash. The only difference beeing that during a crash the rotor(s) become(s) blocked altogether.

Taking the tail rotor of, the cause of the vibration was seen to be some disbalance of the main rotor, which shaft heavily shook already at low speeds. Using bluetag - and a lot of patience in finding the right amount and position on one of the blades - the unbalance was cured and the main shaft and the tail became stable.

But I am not sure whether this is due to a diminished lift for the blade where the bluetag was stuck to, or to a the mass which was added to that blade

But putting the tail rotor blades on again the problem appeared to have been cured.

Hope this helps for you too.