Percona xtrabackup Prepare Fails

Question

I made a backup and according to Percona it completed successfully. Now I'm trying to prepare it and having the following issue:

./xtrabackup_55  --defaults-file="/etc/my.cnf"  --defaults-group="mysqld" --prepare --target-dir=/backup/unprepared/2013_05_21_21_31_41_full
xtrabackup: cd to /backup/unprepared/2013_05_21_21_31_41_full
xtrabackup: This target seems to be not prepared yet.
xtrabackup: No valid checkpoint found.
xtrabackup: Error: xtrabackup_init_temp_log() failed.

I've been searching for answers for quite a while and not finding anything conclusive. Any advice on how to resolve this?

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Edit: per comment below, here is requested info:

xtrabackup_55 version 2.0.3 for Percona Server 5.5.16 Linux (x86_64) (revision id: 470)

cat /etc/my.cnf|grep innodb
innodb_data_home_dir = /data/mysql_data/
innodb_data_file_path = ibdata1:2000M;ibdata2:10M:autoextend
innodb_log_group_home_dir = /var/mysql/data/
innodb_file_per_table
innodb_buffer_pool_size = 5G
innodb_additional_mem_pool_size = 40M
innodb_log_file_size = 1G
innodb_log_buffer_size = 16M
innodb_log_block_size = 512
innodb_flush_log_at_trx_commit = 1
innodb_lock_wait_timeout = 50
innodb_thread_concurrency = 8
innodb_doublewrite_file = /var/mysql/innodb_doublewrite.buff
innodb_flush_method = ALL_O_DIRECT