Actually… the content of the parent kirby was duplicated on the root of this subfolder kirby. There were 0_test/, error/ and home/ folders there, plus a site.txt file.
Deleting all of these resulted in my problem being solved.
How did these files and folders reached that subfolder and how were they preventing regular navigation, escapes me and puzzles me.
If you have any idea I’d be great to understand.
Thank you