Hello Marcin,
Thankyou for your query. Would you tell us how the backups are working ? Are you having any skipped files issue ?
Since when have you been having this problem ? Is this a cluster ?
Have you checked the application and system logs for any apparent errors ?
NOTE : If we do not receive your reply within two business days, this post would be marked ‘assumed answered’ and would be moved to ‘answered questions’ pool.