Before I go into detail that may not help, can you humor me by manually creating your own ‘test.txt’ file text file with the content “OK” under your website in the ./well-known/acme-challenge path? Then rename the file to just “test” instead of “test.txt” - in Windows Explorer you need to show file extensions to do that rename.
I (and you) should then be able to get to it as http://video.leylan.com/.well-known/acme-challenge/test
This would prove:
- the files are being written to the expected path
- extensionless text files work and are served as text
- that the 404 handler for Express is not intercepting the request
- that externally the IP bound for Express is OK