MySQL my.cnf: innodb_log_file_size is missing
Login to mysql and run this

SHOW GLOBAL VARIABLES LIKE 'innodb_log_file_size';


or this

SELECT variable_value FROM information_schema.global_variables
WHERE variable_name = 'innodb_log_file_size';


This will give you the size mysqld has. The default for MySQL 5.5 is 5M.

Whatever number that turns up should be placed in my.cnf. If it is not set, then place this

[mysqld]
innodb_log_file_size = (Whatever the filesize of ib_logfile0 is)


Try out xtrabackup after adding this to my.cnf.