The S3 plugin v0.10.12 settings are lost every time Jenkins restarts. Looking at the logs I find the following errors:
My S3 region is eu-west-2
The following messages are found in the logs:
com.amazonaws.regions.Region in file:/var/lib/jenkins/plugins/aws-java-sdk/WEB-INF/lib/aws-java-sdk-core-1.11.264.jar might be dangerous, so rejecting; see https://jenkins.io/redirect/class-filter/ Feb 05, 2018 11:26:25 AM WARNING hudson.model.Descriptor save Failed to save /var/lib/jenkins/hudson.plugins.s3.S3BucketPublisher.xml java.lang.UnsupportedOperationException: Refusing to marshal com.amazonaws.regions.Region for security reasons; see https://jenkins.io/redirect/class-filter/