Four months ago I have contacted the support and they told me it was a problem on their side, they have temporarily fix it (for the current commit).
Today I tried again
- I deleted the gh-pages branch on github
git push origin --delete gh-pages
- I deleted the gh-pages branch on local
git branch -D gh-pages
- I reinitialized git
- I recreated the branch on local
git branch gh-pages
- I pushed the gh-pages branch to github
git push origin gh-pages
Works fine, I can finally update my files on the page.