Stuck in challenge validation http-01

I’m using IIS 8 with acmesharp.

there is no redirection http -> https

the “challenge link” is visible from internet (in the .well-known/acme-challenge path) but the status stays (according to my acmesharp client) pending for 24h.

From here I don’t know how/where investigate more.

Thanks in advance for any clue

I found the answer here :