backup transport errors : strange errors

Hello, I configured WHM to backup automatically user accounts and to put them on another server using FTP. Usually, it works perfectly and I receive an email saying "the backup process completed" but *sometimes*, I also receive an email "Backup transport errors" which says that some files couldn't be moved to the FTP destination. The strange thing is the following : the files which cannot be moved to the FTP destination are not always the same, but I didn't modify anything in the WHM configuration... (and there is always some gigas of free space on the FTP server...) I cannot understand why this problem arises. Could you help me ? Here are some of the last log files "transport errors encountered" sent to me (in french) : ======= july 7 ======= Impossible d"envoyer " /backup/2015-07-07/accounts/account1.tar.gz " " la destination " myftpserver.com " Impossible d"envoyer " /backup/2015-07-07/accounts/account2.tar.gz " " la destination " myftpserver.com " Impossible d"envoyer " /backup/2015-07-07/accounts/account3.tar.gz " " la destination " myftpserver.com " Impossible d"envoyer " /backup/2015-07-07/accounts/account4.tar.gz " " la destination " myftpserver.com " ======= july 9 ======= Impossible d"envoyer " /backup/2015-07-09/accounts/account4.tar.gz " " la destination " myftpserver.com " ======= july 11 ======= Impossible d"envoyer " /backup/2015-07-11/accounts/account3.tar.gz " " la destination " myftpserver.com " Impossible d"envoyer " /backup/2015-07-11/accounts/account4.tar.gz " " la destination " myftpserver.com " ======= july 14 ======= Impossible d"envoyer " /backup/2015-07-14/accounts/account2.tar.gz " " la destination " myftpserver.com " Impossible d"envoyer " /backup/2015-07-14/accounts/account3.tar.gz " " la destination " myftpserver.com " Impossible d"envoyer " /backup/2015-07-14/accounts/account4.tar.gz " " la destination " myftpserver.com " =====================