DownloadArtifact cannot establish connection

I Have a pretty basic pipeline running on my self-hosted agent:

trigger:
- main

pool:
  name: '3A'
  demands: 
  - Produkt -equals 3A-Editor

steps:
  - checkout: self #pretty redundant, just to show a second step
  - task: DownloadPipelineArtifact@2
    inputs:
      buildType: 'specific'
      project: '65189d0a-e547-4e24-9be0-f5c1ef304db9'
      definition: '324'
      buildVersionToDownload: 'latest'
      artifactName: 'templates'
      targetPath: '$(Build.SourcesDirectory)/gdb'

That run for ages without any problems, but since a week or so the DownloadPipelineArtifact-step throws an error:

Es konnte keine Verbindung hergestellt werden, da der Zielcomputer die Verbindung verweigerte.

which indicates a connection-error. However I'm not even sure which computer want to connect to which here.

The checkout-step runs successfully, so I expect the pipeline is able to run jobs on that agent.

I used the UI for the download-step to chose the correct project and pipeline, so the above IDs should also be okay.