Scheduled backup fails

I have created a scheduled backup. It was working fine but now I have a problem while 3 virtual server backups are uploaded via FTP to another server. All another virtual servers backups successfully. There is a problem just with these 3 (they are the biggest ones, each server backup is about 10 Gb).
Here is the backup log:

Backup failed! See the progress output above for the reason why. Total backup time was 9 hours, 58:21 minutes.
Virtual servers that failed :
Sent by Virtualmin at: https://xxxxx.yyy:10000

Uploading archive to FTP server…
… upload failed! initial connection failed : ransfer complete

… completed in 2 hours, 13:23 minutes

Archive with a backup is created on FTP server (.tar.gz) and I can download and extract files from there, but no other files are created (.tar.gz.dom and A backup log is strange: there is “failed” and “complete” words at the same time and no letter “T” in “Transfer complete” words, just “ransfer complete”.
Is it a bug?

I noticed that backup process uploads .tar.gz file 3 times. It transfers 100% and starts upload again with archive file overwriting.

Can you ftp to the receiving server? Does it have enough disk space?

Yes, there are much space available. The backups are making every day. No problem with space.

I have tried to make a backup to another FTP server. All was completed successfully. Problem FTP server runs Proftpd.

I configured Proftpd with such params:
“AllowStoreRestart on”
“AllowRetrieveRestart on”
and it doesn’t helped.

Backup file is uploading to 100% and upload starts from the beggining, This repeates 3 times and then error.