Comment 18 for bug 1278529

I have analyzed the situation in debugger and it is caused by following code in connection.py, line 540, function S3Connection.lookup() :

        try:
            bucket = self.get_bucket(bucket_name, validate, headers=headers)
        except:
            bucket = None
        return bucket

In my case, the original muted expression was S3ResponseError: S3ResponseError: 403 Forbidden that clearly explained where is the problem.
Until this is patched properly, you can change the code into:

        try:
            bucket = self.get_bucket(bucket_name, validate, headers=headers)
        except:
            raise
        return bucket

to get at least proper error message.