[Sony Xperia XA2 ROMs, Kernels, Recoveries, & Other] markusXA2:
Xperia XA2 backup fails on /data/data with createTarFork() ERROR 255

I want to install Lineage (my first time) on a brand new XA2, after a backup of the device. I work on Linux.

After fastboot boot twrp-3.2.3-2-pioneer.img I first got into a boot-loop saying

Your device software can't be checked for corruption. Please lock the bootloader

This somehow stoped and now only appears exactly once, after which TWRP starts.

I proceed readonly and want to backup.

Backup of the last item "Data (excl. storage)" always fails with createTarFork() ERROR: 255

I:addFile '/data/data' including root: 0
==> set selinux context: u:object_r:system_data_file:s0
failed to lookup tar policy for '/data/data' - '5ecc93d5fa65fd49'
I:Error adding file '/data/data' to '/sdcard1/TWRP/BACKUPS/CQCQCQCQ/1970-01-02--03-53-38_501A13123/data.ext4.win'
Error creating backup.
I:ERROR tarList for thread ID 0
Error creating backup.
I:InfoManager saving '/sdcard1/TWRP/BACKUPS/CQCQCQCQ/1970-01-02--03-53-38_501A13123/data.info'
createTarFork() process ended with ERROR: 255
Backup Failed. Cleaning Backup Folder.

Device info:

$ fastboot getvar all
(bootloader) unlocked:yes
(bootloader) off-mode-charge:0
(bootloader) charger-screen-enabled:0
(bootloader) battery-soc-ok:yes
(bootloader) battery-voltage:4315
(bootloader) version-baseband:1311-2845_50.1.A.13.123
(bootloader) version-bootloader:1310-0301_X_Boot_SDM630_LA1.0_O_123
(bootloader) variant:SDM EMMC
(bootloader) partition-type:userdata:ext4
(bootloader) partition-size:userdata: 0x500000000
(bootloader) partition-type:system_b:ext4
(bootloader) partition-size:system_b: 0xB5000000
(bootloader) has-slot:modem:yes
(bootloader) has-slot:system:yes
(bootloader) current-slot:_b
(bootloader) has-slot:boot:yes
(bootloader) slot-retry-count:b:6
(bootloader) slot-unbootable:b:no
(bootloader) slot-successful:b:yes
(bootloader) slot-retry-count:a:7
(bootloader) slot-unbootable:a:no
(bootloader) slot-successful:a:no
(bootloader) slot-count:2
(bootloader) secure:no
(bootloader) product:H3113
(bootloader) serialno:CQCQCQCQ
(bootloader) max-download-size:536870912
(bootloader) kernel:uefi

Current slot "b" is fine?

What other information is useful?

The backup log starts with 700 lines, of which some are below.

Why is the date 1970?

The phone date is 4 December 2018.

The phone never had an internet connection, after a factory reset (caused by the boot-loop)

Starting TWRP 3.2.3-2-c757a664 on Fri Jan 2 08:50:11 1970
(pid 545)
RECOVERY_SDCARD_ON_DATA := true
I:Lun file '/sys/class/android_usb/android0/f_mass_storage/lun0/file' does not exist, USB storage mode disabled
TW_INCLUDE_CRYPTO := true
......
I:TW_EXCLUDE_MTP := true
I:LANG: en
I:AB_OTA_UPDATER := true
Starting the UI...
setting DRM_FORMAT_RGBX8888 and GGL_PIXEL_FORMAT_RGBX_8888
cannot find/open a drm device: No such file or directory
........
I:File Based Encryption is present
e4crypt_initialize_global_de
calling retrieveAndInstallKey
Key exists, using: /data/unencrypted/key
Using Keymaster HAL: 3 from QTI for encryption. Security level: TRUSTED_ENVIRONMENT, HAL: android.hardware.keymaster@3.0::IKeymasterDevice/default
begin failed, code -62
Upgrading key in memory only: /data/unencrypted/key
.......
Preparing: /data/vendor_de/0
Failed to prepare /data/vendor_de/0
Failed to prepare user 0 storage
e4crypt_init_user0 returned fail
I:Loading page select_storage
I:Loading page select_language
I:Set page: 'clear_vars'
I:Set page: 'main2'
I:Switching packages (TWRP)
No file contexts for SELinux
I:Mount: Unable to find partition for path '/cache'
I:Could not check /cache/recovery SELinux contexts, using /sbin/teamwin instead which may be inaccurate.
Full SELinux support is present.
I:Mount: Unable to find partition for path '/cache'
......
I:operation_start: 'Nandroid'
Updating partition details...
I:Data backup size is 999MB, free: 18948MB.
I:Unable to mount '/usb_otg'
I:Actual block device: '', current file system: 'vfat'
...done
I:Backup Name is: '1970-01-02--03-53-38_501A13123'
I:Backup_Folder is: '/sdcard1/TWRP/BACKUPS/CQCQCQCQ/1970-01-02--03-53-38_501A13123'
I:Calculating backup details...
* Total number of partitions to back up: 1
* Total size of all data: 999MB
* Available space: 8511MB
[BACKUP STARTED]
* Backup Folder: /sdcard1/TWRP/BACKUPS/CQCQCQCQ/1970-01-02--03-53-38_501A13123
Invalid encryption mode 'aes-256-xts:aes-256-heh'
Backing up Data (excl. storage)...
Backups of Data do not include any files in internal storage such as pictures or downloads.
Invalid encryption mode 'aes-256-xts:aes-256-heh'
I:Creating backup...
I:Creating tar file '/sdcard1/TWRP/BACKUPS/CQCQCQCQ/1970-01-02--03-53-38_501A13123/data.ext4.win'
I:addFile '/data/bootchart' including root: 0
==> set selinux context: u:object_r:bootchart_data_file:s0
found policy '/data/bootchart' - '1DK' - '03787db07d4e143b'
.....

Thanks