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 get root-permissions, which means that you may get errors with 'amcheck' while 'amdump' runs through without problems.