Already added Issue at GitHub for Kirby Panel: https://github.com/getkirby/panel/issues/380
If project.txt is renamed to Project.txt in Content folder (in my case there was different name), textarea content in Panel is not showing, therefore user can not edit content in Panel.
It happened, because I wanted to change previously choosed template to different. So I changed *.txt filename to other templates name, but first letter wrote capitalized. On my Win / Chrome everything was ok even if first letter was capitalized, but on friends Mac / Safari textarea content was not showing.