ERROR: target path already exists

Recently my backups started failing with this error: “target path already exists”. Server OS is Linux Mint 19.3 kernel 5.4.x (backup on mirrored btrfs HDDs) and UrBackup server version is 2.4.13, all clients run Win 10.

UrBackup just did an incremental file backup of “X”.

Report:
( 8 infos, 1 warning, 2 errors)

2020-12-29 10:39:03(info): Starting scheduled incremental file backup…
2020-12-29 10:39:17(info): Selected no components to backup
2020-12-29 10:39:17(info): Scanning for changed hard links on volume of “D:”…
2020-12-29 10:39:17(info): Indexing of “XXX” done. 9 filesystem lookups 4988 db lookups and 5 db updates
2020-12-29 10:39:17(info): Indexing of “YYY” done. 1 filesystem lookups 909 db lookups and 0 db updates
2020-12-29 10:39:17(info): X: Loading file list…
2020-12-29 10:39:18(info): X: Calculating file tree differences…
2020-12-29 10:39:18(info): X. Creating snapshot…
2020-12-29 10:39:18(warning): Creating new snapshot failed (Server error) “Create a snapshot of ‘/mnt/backup/urbackup/X/201228-1036’ in ‘/mnt/backup/urbackup/X/201229-1039’”
2020-12-29 10:39:18(error): Creating empty filesystem failed (Server error) "ERROR: target path already exists: /mnt/backup/urbackup/X/201229-1039
1970-01-01 01:00:00(info):
2020-12-29 10:39:23(error): Backup had an early error. Deleting partial backup.

Backup of one of the clients would succeed on second attempt others would keep failing on every further attempt. Note the bizarre line dated January 1st, 1970! This line appeared in some of the reports of failed backups.

UrBackup just did an incremental file backup of “X”.

Report:
( 19 infos, 1 warning, 0 errors)

2020-12-29 11:19:24(info): Starting scheduled incremental file backup…
2020-12-29 11:19:39(info): Selected no components to backup
2020-12-29 11:19:39(warning): Restarting shadow copy of D:\ because it was started by this server
2020-12-29 11:19:39(info): Scanning for changed hard links on volume of “D:”…
2020-12-29 11:19:39(info): Indexing of “XXX” done. 2 filesystem lookups 4995 db lookups and 1 db updates
2020-12-29 11:19:39(info): Indexing of “YYY” done. 1 filesystem lookups 909 db lookups and 0 db updates
2020-12-29 11:19:39(info): X: Loading file list…
2020-12-29 11:19:39(info): X: Calculating file tree differences…
2020-12-29 11:19:39(info): X: Creating snapshot…
2020-12-29 11:19:40(info): X: Deleting files in snapshot… (3)
2020-12-29 11:19:40(info): X: Deleting files in hash snapshot…(3)
2020-12-29 11:19:40(info): X: Calculating tree difference size…
2020-12-29 11:19:40(info): X: Linking unchanged and loading new files…
2020-12-29 11:19:40(info): Waiting for file transfers…
2020-12-29 11:19:46(info): Waiting for file hashing and copying threads…
2020-12-29 11:19:47(info): Writing new file list…
2020-12-29 11:19:47(info): All metadata was present
2020-12-29 11:19:48(info): Transferred 44.0207 MB - Average speed: 53.6733 MBit/s
2020-12-29 11:19:54(info): Time taken for backing up client X: 30s
2020-12-29 11:19:54(info): Backup succeeded

Server restart after kernel update resolved the issue, so I’m not sure if restart or kernel update solved the problem.