Hi,
I saw multiple subjects about this :
https://community.letsencrypt.org/t/unable-to-verify-file-which-was-uploaded-in-acme-challenge/57714
https://community.letsencrypt.org/t/sslforfree-com-fails-to-verify-the-uploaded-files/33691
in my case, on IIS, it was a configuration to do on IIS to accept files without extension :
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
<staticContent>
<mimeMap fileExtension="." mimeType="text/xml" />
</staticContent>
</system.webServer>
</configuration>