[Samsung Galaxy S21 ROMs, Kernel] okwhateverok:
The log file is attached to this post and the nandroid backup location is the internal sd card.

Backing up keyrefuge...
I:Creating backup...
I:Creating tar file '/data/media/0/TWRP/BACKUPS/SM-G991B/2021-12-07--22-21-12_RP1A200720012G991BXXS3AUJ7/keyrefuge.f2fs.win'
I:addFile '/keyrefuge/metadata_encryption' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/metadata_encryption/key' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/metadata_encryption/key/version' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/metadata_encryption/key/secdiscardable' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/metadata_encryption/key/stretching' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/metadata_encryption/key/keymaster_key_blob' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/metadata_encryption/key/encrypted_key' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/metadata_encryption/key/integrity' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/metadata_encryption/key/rot' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/unencrypted' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/unencrypted/key' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/unencrypted/key/version' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/unencrypted/key/secdiscardable' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/unencrypted/key/stretching' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/unencrypted/key/keymaster_key_blob' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/unencrypted/key/encrypted_key' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/unencrypted/key/integrity' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/unencrypted/mode' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/unencrypted/ref' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/misc' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/misc/HQxNjCAAAAwiDruThULsGprIr+WFM0zM' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/misc/HQxNjCAAAAwiDruThULsGprIr+WFM0zM/IdUpXBAAAAQCIHdy7M7jhe46eyxaUbwA' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/misc/HQxNjCAAAAwiDruThULsGprIr+WFM0zM/IdUpXBAAAAQCIHdy7M7jhe46eyxaUbwA/GfJNiCAAAAAdKRDI8ShAHwNoKJHfsFeT' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/misc/HQxNjCAAAAwiDruThULsGprIr+WFM0zM/IdUpXBAAAAQCIHdy7M7jhe46eyxaUbwA/GfJNiCAAAAAdKRDI8ShAHwNoKJHfsFeT/PjYzIBAAAAAILDiPwAvENkkqorDBZfui' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/misc/HQxNjCAAAAwiDruThULsGprIr+WFM0zM/IdUpXBAAAAQCIHdy7M7jhe46eyxaUbwA/GfJNiCAAAAAdKRDI8ShAHwNoKJHfsFeT/PjYzIBAAAAAILDiPwAvENkkqorDBZfui/I5VYtAAAAAAlkTcNTaMHrMtKRek4j5oc' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:addFile '/keyrefuge/misc/HQxNjCAAAAwiDruThULsGprIr+WFM0zM/IdUpXBAAAAQCIHdy7M7jhe46eyxaUbwA/GfJNiCAAAAAdKRDI8ShAHwNoKJHfsFeT/PjYzIBAAAAAILDiPwAvENkkqorDBZfui/I5VYtAAAAAAlkTcNTaMHrMtKRek4j5oc/N6sEWAAAAAQNZ3lEyRkNy64mcH8QoGSq' including root: 0
==> set selinux context: u:object_r:keyrefuge_data_file:s0
I:Error adding file '/keyrefuge/misc/HQxNjCAAAAwiDruThULsGprIr+WFM0zM/IdUpXBAAAAQCIHdy7M7jhe46eyxaUbwA/GfJNiCAAAAAdKRDI8ShAHwNoKJHfsFeT/PjYzIBAAAAAILDiPwAvENkkqorDBZfui/I5VYtAAAAAAlkTcNTaMHrMtKRek4j5oc/N6sEWAAAAAQNZ3lEyRkNy64mcH8QoGSq' to '/data/media/0/TWRP/BACKUPS/SM-G991B/2021-12-07--22-21-12_RP1A200720012G991BXXS3AUJ7/keyrefuge.f2fs.win'
Error creating backup.
I:ERROR tarList for thread ID 0
Error creating backup.
I:InfoManager saving '/data/media/0/TWRP/BACKUPS/SM-G991B/2021-12-07--22-21-12_RP1A200720012G991BXXS3AUJ7/keyrefuge.info'
createTarFork() process ended with ERROR: 255
Backup Failed. Cleaning Backup Folder.