You need to get the end point and region from your s3 provider if I remember in this case is someone else than aws , which it appears you were trying to connect to. You need to learn those values from your provider
EDIT :-
found this on the contabo help website
Can I mount S3 Object Storage files to my personal computer?
It is possible to mount S3 Object Storage to your VPS/VDS or personal computer. This way you can access the files and folders of your S3 Object Storage like a local drive which makes usage more convenient. The methods to set this up are different for Windows and Linux but there are some things you will need on both Windows and Linux:
Contabo's S3 URL.
The Object Storage's access key.
The Object Storage's secret key.
You can find the S3 URL on the buckets overview page in the "Bucket URL" column.
which explains where the end point is (Contabo’s S3 URL) but doesn’t seem to mention the region don’t know if that is an issue for virtualmin or not