Backup failed because of disk problems

I have a problem with backup. Please, advise something.

Urbackup server: Ubuntu 16.04 LTS / Urbackup 2.0.30 / btrfs
Urbackup client :Ubuntu 14.04.4 LTS / Urbackup client v. 2.0.30 /ext4

client log;
0-1472957215-Logical volume "urbackup_snap_b02490ac5c567233014ca408d8dc9e4fead6facdeec213a1" created 0-1472957215-Indexing of "vmail" done. 17223 filesystem lookups 0 db lookups and 0 db updates 0-1472957215-storage.atlas.local[vmail]: Loading file list... 0-1472957217-storage.atlas.local[vmail]: Calculating file tree differences... 0-1472957224-storage.atlas.local[vmail]: Creating snapshot... 0-1472957227-storage.atlas.local[vmail]: Deleting files in snapshot... (14231) 0-1472957346-storage.atlas.local[vmail]: Deleting files in hash snapshot... 0-1472957372-storage.atlas.local[vmail]: Calculating tree difference size... 0-1472957374-storage.atlas.local[vmail]: Linking unchanged and loading new files... 2-1472957401-Error writing to file "/Storage/BACKUP/urbackup/storage.atlas.local[vmail]/160904-0543/vmail/ decorative/dovecot.index$ 2-1472957401-Storing file to "/Storage/BACKUP/urbackup/storage.atlas.local[vmail]/160904-0543/vmail/ decorative/dovecot.index.log" $ 0-1472957661-Waiting for file transfers... 0-1472957667-Waiting for file hashing and copying threads... 0-1472957668-Writing new file list... 2-1472957679-Fatal error during backup. Backup not completed 0-1472957680-Transferred 6.77908 GB - Average speed: 128.943 MBit/s 2-1472957681-FATAL: Backup failed because of disk problems (see previous messages) 0-1472957682-Time taken for backing up client storage.atlas.local[vmail]: 11m 2-1472957682-Backup failed

Server log:
2016-09-04 05:50:01: ERROR: Error writing to file "/Storage/BACKUP/urbackup/storage.atlas.local[vmail]/160904-0543/vmail/ decorative/dovecot.index.log" -2 2016-09-04 05:50:01: ERROR: Storing file to "/Storage/BACKUP/urbackup/storage.atlas.local[vmail]/160904-0543/vmail/ decorative/dovecot.index.log" failed 2016-09-04 05:54:39: ERROR: Fatal error during backup. Backup not completed 2016-09-04 05:54:41: ERROR: FATAL: Backup failed because of disk problems (see previous messages) 2016-09-04 05:54:42: ERROR: Backup failed