Android : Error when using AWS Android S3 SDK: Source not found - The JAR of this class file belongs to container 'Android private libraries'

I'm working on an Android App with AWS Android SDK, especifically using S3, and I started receiving the following error:

Source not found - The JAR of this class file belongs to container 
'Android private libraries' which does not allow modifications to source attachments
on its entries.

that happens on the line

s3Client = new AmazonS3Client(credentials);

when I try to create a new client to connect to S3.

I had to replace an old version of the SDK (1.7.1.1) with a newer version (2.1.0) in order to use PersistableDownload class to pause/resume downloads. I don't understand why is this happening since I'm not trying to change the library. What I remember doing:

- Removed the old library
- Added the new one
- Removed and added again several imports on the start of .java files
- Using an older device for the weekend (android API level 8)

Here is a extend part of the code. That might help.

AWSCredentials credentials = new BasicAWSCredentials(ACCESS_KEY_ID, SECRET_KEY);

Region region = Region.getRegion(regions);

AmazonS3Client s3Client = null;
try {
    s3Client = new AmazonS3Client(credentials);
    s3Client.setRegion(region);
} catch (Exception e) {
    e.printStackTrace();
}
return s3Client;

Any help is aprpreciated to solve this. Thanks!