the reason the s3 upload/backup does not work is the region that needs to be set to fr-par (or nl-ams) which can only be set by circumventing discourse’s SiteSetting validation:
(via ./launcher enter app then rails c)
SiteSetting.find_by(name: 's3_region').update_attribute(:value, 'fr-par')
after this change uploads and backups work to scaleway object storage.
Scaleway documentation on Object Storage
of course, this is a workaround and once you reset or change this site setting via Web Admin you cannot set it back to a working state (unless using rails console again)
I guess the aws/s3 Client all allow explicitly setting a region string (as opposed to the current state of the webadmin)
It is also kind of misleading in case of the “EU (Paris)” dropdown value in discourse, since this refers to the aws naming scheme eu-west-3 (or sth. like that) and not the expected value for scaleway.