@Remi-Gau I think I just fixed it with the following steps that might also need to be applied to other repos:
log into circleci app and select the failing repo
click on project settings (upper right)
click "SSH keys"
remove the deploy key by clicking on the small x (see also screenshot)
re-add a new deploy key by clicking the button that appears when no deploy keys are present anymore (this gets done automatically, no command-line ssh action required)
Describe your problem in detail.
god knows why it cannot checkout the repo...
Describe what you expected.
No response