Meaning and mitigation of: "WARNING: Failed to open x / y files"

[HaleTom]

I saw the following output:

% mkdir /tmp/new-cache
% sudo btrbk --override 'cache_dir=/tmp/new-cache' extents /media/ssd/btrbk/home-var/*
WARNING: Failed to open 805329 / 805335 files
WARNING: Failed to open 805325 / 805331 files
WARNING: Failed to open 807545 / 807551 files
WARNING: Failed to open 809582 / 809588 files
WARNING: Failed to open 809457 / 809463 files
WARNING: Failed to open 810760 / 810766 files
WARNING: Failed to open 2640077 / 2640085 files
WARNING: Failed to open 3300898 / 3300906 files
TOTAL       EXCLUSIVE   SUBVOL
333.72 GiB  332.47 GiB  /media/ssd/btrbk/home-var/home-var.20250206T1903
350.71 GiB   76.31 GiB  /media/ssd/btrbk/home-var/home-var.20250314T1600
277.82 GiB  271.52 MiB  /media/ssd/btrbk/home-var/home-var.20250422T1351
277.85 GiB    2.54 MiB  /media/ssd/btrbk/home-var/home-var.20250424T1045
277.85 GiB    3.39 MiB  /media/ssd/btrbk/home-var/home-var.20250424T1113
277.85 GiB   33.18 MiB  /media/ssd/btrbk/home-var/home-var.20250424T1833
277.85 GiB    2.89 MiB  /media/ssd/btrbk/home-var/home-var.20250425T1203
277.85 GiB    3.91 MiB  /media/ssd/btrbk/home-var/home-var.20250425T1503

Union (8 subvolumes):  687.37 GiB

Questions:

• What circumstances would cause files to not be able to be opened by root? (btrfs scrub is clean)
• How would I fix things so that the warnings no longer appear?
• Could the UUID be listed against each warning? (eg to be able to delete the cache after fixing it)