Can you actually add the bucket to the credentials just to try it out?
Also, can you check that this snippet works for you (with your creds):

import boto3
import json
import six

key = ""
secret = ""
host = "our_host.com"
bucket_name = "bucket"
profile = None
filename = "test"
data = {"test": "data"}

boto_session = boto3.Session(aws_access_key_id=key, aws_secret_access_key=secret, profile_name=profile)
endpoint = "https://" + host
boto_resource = boto_session.resource("s3", region_name=None, endpoint_url=endpoint)
bucket = boto_resource.Bucket(bucket_name)
bucket.put_object(Key=filename, Body=six.b(json.dumps(data)))