I built a new repo with same .htaccess : Index of /villonCycle4/seance2/templates
and get the : "Unable to load Project Template Data." error
This also occurs on all previous template repositories that previously worked fine.
.htaccess contents :
Header set Access-Control-Allow-Origin "*"
I should also note that the original URL include the templates/ directory, but App Inventor will automatically add this so it's unnecessary to include it in the URL.
Thank you Ewan, this solves the problem.
(Repositories are great for teaching. I them as canvas, intermediate app versions, so as not to loose students on the road. They also help make teachers feel safe and provide them with external support.)
I also noticed that there may be some delay on repository updates. For example when you delete or change an existing template. This is not a problem once you know (I will reread the doc).