Amrecover fails with a "amrecover: Can't read file header" error

When running amrecover from this error occurs:

On console:
Load tape TEST-1 now
Continue [?/Y/n/s/t]? y
EOF, check amidxtaped.<timestamp>.debug file on vmservercomm.zmanda.com.
amrecover: short block 0 bytes
UNKNOWN file
amrecover: Can't read file header
extract_list - child returned non-zero status: 1
Continue [?/Y/n/r]? n

In debug file:
amrestore: WARNING: not at start of tape, file numbers will be offset
amrestore:   0: reached end of information
amidxtaped: time 0.250: amrestore terminated normally with status: 1
amidxtaped: time 0.256: rewinding tape ...
amidxtaped: time 0&4: done

Solution: Make sure that the amandabackup user is added to the disk group:

disk:x:6:root,amanda