With a custom endpoint, saving stashes and artifacts works but unstashing does not, as it uses s3.amazonaws.com:

{{org.jclouds.aws.AWSResponseException: request GET https://s3.amazonaws.com/jenkins-artifacts/***/master/237/stashes/build-artifacts.tgz HTTP/1.1 failed with code 403, error: AWSError{requestId='148VY12NC9GS8X21', requestToken='****', code='InvalidAccessKeyId', message='The AWS Access Key Id you provided does not exist in our records.', context='

'}}}
    at org.jclouds.aws.handlers.ParseAWSErrorFromXmlContent.handleError(ParseAWSErrorFromXmlContent.java:76)
Caused: org.jclouds.rest.AuthorizationException: The AWS Access Key Id you provided does not exist in our records.
    at org.jclouds.aws.handlers.ParseAWSErrorFromXmlContent.refineException(ParseAWSErrorFromXmlContent.java:123)
    at org.jclouds.s3.handlers.ParseS3ErrorFromXmlContent.refineException(ParseS3ErrorFromXmlContent.java:97)
    at org.jclouds.aws.handlers.ParseAWSErrorFromXmlContent.handleError(ParseAWSErrorFromXmlContent.java:90)
    at org.jclouds.http.handlers.DelegatingErrorHandler.handleError(DelegatingErrorHandler.java:65)
    at org.jclouds.http.internal.BaseHttpCommandExecutorService.shouldContinue(BaseHttpCommandExecutorService.java:138)
    at org.jclouds.http.internal.BaseHttpCommandExecutorService.invoke(BaseHttpCommandExecutorService.java:107)
    at org.jclouds.rest.internal.InvokeHttpMethod.invoke(InvokeHttpMethod.java:91)
    at org.jclouds.rest.internal.InvokeHttpMethod.apply(InvokeHttpMethod.java:74)
    at org.jclouds.rest.internal.InvokeHttpMethod.apply(InvokeHttpMethod.java:45)
    at org.jclouds.rest.internal.DelegatesToInvocationFunction.handle(DelegatesToInvocationFunction.java:156)
    at org.jclouds.rest.internal.DelegatesToInvocationFunction.invoke(DelegatesToInvocationFunction.java:123)
    at com.sun.proxy.$Proxy271.getObject(Unknown Source)
    at org.jclouds.s3.blobstore.S3BlobStore.getBlob(S3BlobStore.java:235)
    at org.jclouds.blobstore.internal.BaseBlobStore.getBlob(BaseBlobStore.java:217)
    at io.jenkins.plugins.artifact_manager_jclouds.JCloudsArtifactManager.unstash(JCloudsArtifactManager.java:296)
    at org.jenkinsci.plugins.workflow.flow.StashManager.unstash(StashManager.java:155)
    at org.jenkinsci.plugins.workflow.support.steps.stash.UnstashStep$Execution.run(UnstashStep.java:78)
    at org.jenkinsci.plugins.workflow.support.steps.stash.UnstashStep$Execution.run(UnstashStep.java:65)
    at org.jenkinsci.plugins.workflow.steps.SynchronousNonBlockingStepExecution.lambda$start$0(SynchronousNonBlockingStepExecution.java:47)
    at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at java.base/java.lang.Thread.run(Thread.java:829)

Configuration in Amazon S3 Bucket Access settings looks like this:

S3 bucket name: jenkins-artifacts

Custom endpoint: gos3.io

Custom signing region: eu-central-1

Use Path Style URL: yes

Disable Session Token: yes

Amazon Credentials - Region: EU (Frankfurt)

Amazon Credentials: configured with access key and secret key, global scope, and no IAM configuration