Why does amcheck fail while amdump succeeds?

If you use GNUtar as backup-binary, amdump uses a wrapper called runtar which is set suid root at install-time:

  # ll /usr/local/libexec/runtar
  -rwsr-x---  1 root disk 13K Oct 28 11:59 /usr/local/libexec/runtar

amcheck does not use that wrapper, so it does not have root-permissions, which means that you may get errors with amcheck while amdump runs through without problems. You can check for amcheck errors in /tmp/amanda/amcheck* logs on the client. There should be "permission denied" error messages.