"Your flight has been canceled."

One hour and a half into my tracked flight, I suddenly have a voice message about my flight no longer being tracked and that I should close the client…
I have no idea why and what did I do wrong.
Trying to resume tracking doesn’t work, I can tick “resume my flight” but I cannot resume the tracking.

I do not think I will continue using such a touchy tracker.
At least tell users WHY.

Are you sure you were not somehow disconnected from the internet?
Or you might have had two clients open and the one that was not tracking suddenly noticed it was not tracked.
If that is the case you ought to double check and close the inactive client.