Hi Maurizio,
Amazon has 2 authentication schemes, and AWS4-HMAC-SHA256 is one of them (this is also referred to as V4). The LabVIEW Interface for Amazon S3 is using V2, which is the other, older authentication scheme.
Amazon has decided to support only the newer, V4 version in regions deployed after January 30th, 2014.
You can read more about this here: Authenticating Requests (AWS Signature Version 4)
I guess you probably tried to upload your data to the Frankfurt region, which is the only such new region.
Take a look at this list of the available S3 regions and their signature version support.
Unfortunately as V4 authentication is not supported in the LabVIEW Interface for Amazon S3, there is no other solution than using a different region to store your data.
Regards,
Peter