I found the solution here:
https://forum.manjaro.org/t/gui-package-manager-unable-to-lock-database-failed-to-synchronize-databases-failed-to-synchronize-aur-database-no-lock-file-found/114741
Deleted db.lck in /var/tmp/pamac/dbs/
Everything ran as expected, without the warining, and no problems with the updated system.