Letsencrypt timeout when fetching test file on http-01 challenge - timeout

Question

I had to pause my dev for a few months. My Letsencrypt certificate expired in the meantime and there some changes in the libs.
I deleted my Letsencrypt directory (the one whith the certificates inside).
I am using greenlock-express API
Now,I cannot manage to pass the http-01 challenge when obtaining the certificate.
I have no problem to curl the file :
http://myserver.com/.well-known/acme-challenge/hRsScb0ZHcFyaroe73h2lnFcQfCTeDwFhVB8PtKHfYs
But I get errors when running my app :
ERROR in le-sni-auto:
Error: The CA was unable to validate the file you provisioned.
Fetching http://myserver.com/.well-known/acme-challenge/hRsScb0ZHcFyaroe73h2lnFcQfCTeDwFhVB8PtKHfYs: Timeout
I cannot firgure out whet is happenning...
Any idea ?
Edit 170912 - Configuration details
var lex = require('greenlock-express').create({
server: 'staging'
,challenges: {'http-01': require('le-challenge-fs').create({ webrootPath: './letsencrypt/tmp' })}
,store: require('le-store-certbot').create({
configDir: './letsencrypt'
,privkeyPath: ':configDir/live/:hostname/privkey.pem'
,fullchainPath: ':configDir/live/:hostname/fullchain.pem'
,certPath: ':configDir/live/:hostname/cert.pem'
,chainPath: ':configDir/live/:hostname/chain.pem'
,webrootPath: ':configDir/tmp'
,debug: false
})
,approveDomains: approveDomains
});
function approveDomains(opts, certs, cb) {
if (certs) {
opts.domains = certs.altnames;
}
else {
opts.email = 'contact#myserver.com';
opts.agreeTos = true;
opts.domains = [ 'myserver.com'];
}
cb(null, { options: opts, certs: certs });
}
http.createServer(lex.middleware(require('redirect-https')())).listen(80, _ => {
console.log("Listening for ACME http-01 challenges on", this.address());
});
https.createServer(lex.httpsOptions, lex.middleware(app)).listen(443, function () {
console.log("Listening for ACME tls-sni-01 challenges and serve app on", this.address());
});