Welcome to the forum @diogo
You can probably do a Google or other search to find out. I searched on the error only, and found these:
(includes workaround – unclear what cause is)
S3 Explorer Compatible Buckets - Object Storage Server APP
(suggests that the user contact QNAP support)
FWIW Google “qnap” “s3” site:forum.duplicati.com claims about 112 results. Some may help with this.
https://forum.duplicati.com/search?q=qnap%20s3 forum search somehow found far fewer hits.