XtraBackup fails with my.cnf Percona Configuration
the hint is in last error messages as follows

xtrabackup: innodb_log_file_size = 50331648
InnoDB: Error: log file ./ib_logfile0 is of different size 268435456 bytes


my.cnf implicitly refereced by xtrabackup has innodb_log_file_size = 48M (50331648 bytes), but actual log file size is 256MB (as shown your my.cnf)

I think you should specify my.cnf explicitly like this

xtrabackup --defaults-file=/path/to/my.cnf