Failed signature during Backup

Hello , 

i want  a Help with HPe  . I am using 

VM explorer 7.1  withh ESXI 5.5 

some VMs ended the Backup without error but another VM failed with Faild Signature .  i want to know hwere ist the Probleme !Look the image !

  INFO  [5121471] EndAccessVM on host "xxx.Domain" VM xx (vm-13446) successfully terminated
[1776] 14.09.2018 19:42:46,414        ERROR [5121471] BackupExecutor: failed signature
[1776] 14.09.2018 19:42:46,430        ERROR [5121471] Perform ESX (VDDK) Incremental Backup exception: failed signature