Unable to pacman -Syu

[lockheed]

I went to update my Raspberry Pi after a week, and cannot do so. Running pacman -Syu produces the same error, as the following one-package update:
$this->bbcode_second_pass_code('', '# pacman -S lzo
resolving dependencies...
looking for inter-conflicts...
:: lzo and lzo2 are in conflict. Remove lzo2? [y/N] y

Packages (2): lzo2-2.08-1 [removal] lzo-2.08-3

Total Download Size: 0.07 MiB
Total Installed Size: 0.38 MiB
Net Upgrade Size: 0.00 MiB

:: Proceed with installation? [Y/n]
:: Retrieving packages ...
error: could not open file /var/cache/pacman/pkg/lzo-2.08-3-armv6h.pkg.tar.xz.part: Permission denied
error: could not open file /var/cache/pacman/pkg/lzo-2.08-3-armv6h.pkg.tar.xz.part: Permission denied
error: could not open file /var/cache/pacman/pkg/lzo-2.08-3-armv6h.pkg.tar.xz.part: Permission denied
error: could not open file /var/cache/pacman/pkg/lzo-2.08-3-armv6h.pkg.tar.xz.part: Permission denied
error: could not open file /var/cache/pacman/pkg/lzo-2.08-3-armv6h.pkg.tar.xz.part: Permission denied
error: could not open file /var/cache/pacman/pkg/lzo-2.08-3-armv6h.pkg.tar.xz.part: Permission denied
error: could not open file /var/cache/pacman/pkg/lzo-2.08-3-armv6h.pkg.tar.xz.part: Permission denied
error: could not open file /var/cache/pacman/pkg/lzo-2.08-3-armv6h.pkg.tar.xz.part: Permission denied
warning: failed to retrieve some files
error: failed to commit transaction (failed to retrieve some files)
Errors occurred, no packages were upgraded.
synchronizing filesystem...
')

I seem to have enough space, besides rebooting did not help:
$this->bbcode_second_pass_code('', '# df
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/mmcblk0p5 7467008 3491088 3657152 49% /
devtmpfs 242780 0 242780 0% /dev
tmpfs 247000 71532 175468 29% /dev/shm
tmpfs 247000 384 246616 1% /run
tmpfs 247000 0 247000 0% /sys/fs/cgroup
tmpfs 358400 396 358004 1% /tmp
/dev/mmcblk0p1 91962 24934 67028 28% /boot
tmpfs 49400 0 49400 0% /run/user/1000
tmpfs 49400 0 49400 0% /run/user/0
')

[WarheadsSE]

1) thats not an update, that an installation from known indexes.
2) that's a .part file, and is an incomplete download
3) check the actual permissions

[lockheed]

1) I gave installation as an example because it has less files, and the problem and output is exactly the same.
2) /var/cache/pacman/pkg/ remains empty
3) the permissions for this folder are: drwxr-xr-x 1 root root

[WarheadsSE]

Has something remounted root readonly due to errors?

[lockheed]

No, I can write to all locations on root, except for /var/cache/pacman/pkg/

I am using btrfs, too.
/var/cache/pacman/pkg is /'s subvolume, but it is not present in fstab and is being mounted automatically with the rest of root.

[WarheadsSE]

subvolume ..