S3 external storage doesn't works

Ciao *,

short version:
S3 external storage doesn’t works, need “debug” log.

long version:
I’m using NC 12.0.2 on Ubuntu 17.04 zesty (php 7.0, apache2 2.4.25, mysql 5.7.19)
I’m trying to setup an external storage via S3 driver, with a local server (not Amazon AWS) that still use signature version 2.

with command line s3cmd everything is working, for example:

$ s3cmd --no-ssl --access_key=ACCESSKEY --secret_key=SECRETKEY --signature-v2 --host= --host-bucket= mb s3://mybucket/

$ s3cmd --no-ssl --access_key=ACCESSKEY --secret_key=SECRETKEY --signature-v2 --host= --host-bucket= put test.txt s3://mybucket/

but if I try to setup a bucket into “External Storages” page, even with a non-exixting bucket
(Cannot enable Amazon S3 with correct bucket)
with all the configurations compiled correctly, it doesn’t works (RED square)

no error appear, just a red square.

in nextcloud.log:

… method":“GET”,“url”:"/nextcloud/apps/files_external/globalstorages/1?testOnly=true",“message”:"Exception: {“Exception”:“Exception”,“Message”:“The configured bucket name is invalid.”,“Code”:0, …

either with a non existing bucket or with an existing bucket.

I would like to see the command line option that curl send to the server, since that bucket name is valid (from command line s3cmd)

Any clue?
Thank you.