After hours and hours to reset everything and following many answers, the hotline contacted using fitbit app gave me the solution in one minute: cut the wifi, uninstall ecg app, reinstall.
The hotline knew, she said that she raised the topic. This incredible bug is knwon by the developper. This is more than incredible that nobody put pressure on the developper to correct that, it is a well known issue.
And this os one solution for a problem, but we see thouands of posts about this ECG. There is a huge issue on the development side in Fitbit, please google, begin to fire everybody and replace by real engineer.
