Copy-back thread reports an error when is has not done anything

190128 09:34:42 [02]        ...done
190128 09:34:42 [02] Moving ./xtrabackup_master_key_id to /home/krunal.bauskar/percona-xtradb-cluster/target/PXC/mysql-test/var/1/mysqld.2/data//xtrabackup_master_key_id
190128 09:34:42 [02]        ...done
190128 09:34:42 [02] Moving ./ib_buffer_pool to /home/krunal.bauskar/percona-xtradb-cluster/target/PXC/mysql-test/var/1/mysqld.2/data//ib_buffer_pool
190128 09:34:42 [02]        ...done
Error: copy-back thread 0 failed.