pacman -Scc worked.
Then, following with the how-to:
# [ -f /var/lib/pacman/db.lck ] && rm -f /var/lib/pacman/db.lck
# pacman-mirrors -f && pacman -Syyu
::INFO Downloading mirrors from Manjaro
...
::INFO Mirror list generated and saved to: /etc/pacman.d/mirrorlist
:: Synchronizing package databases...
...
:: Proceed with installation? [Y/n]
:: Retrieving packages...
...
(5/5) checking available disk space [############################] 100%
:: Running pre-transaction hooks...
(1/1) Creating Timeshift snapshot before upgrade...
First run mode (config file not found)
Selected default snapshot type: BTRFS
E: System disk not found!
Unable to run timeshift-autosnap! Please close Timeshift and try again. Script will now exit...
error: command failed to execute correctly
error: failed to commit transaction (failed to run transaction hooks)
Errors occurred, no packages were upgraded.