artifact-manager-s3-plugin hardcodes the S3 endpoint URL as:
https://%s.s3.amazonaws.com/
See https://github.com/jenkinsci/artifact-manager-s3-plugin/blob/cca5bc848c1acf59524ae66cf983d0fe6e9a9667/src/main/java/io/jenkins/plugins/artifact_manager_jclouds/s3/S3BlobStore.java#L164
The region must be included for US GovCloud support.
It also must be included for all buckets made after September 30, 2020: https://aws.amazon.com/blogs/aws/amazon-s3-path-deprecation-plan-the-rest-of-the-story/
The URL should be:
https://%s.s3.<region>.amazonaws.com/
Here's a pull request: https://github.com/jenkinsci/artifact-manager-s3-plugin/pull/97