by Alain Raap.
I found the cause of all our backup problems. After I made a change to the InnoDB settings of MariaDB (the buffer pool size was too small), the problem still existed yesterday. Yesterday I uninstalled the tool_excimer plugin, which I suspected that it could interfere the backups, because it was the only plugin that was active during the automated backup. This morning I checked the automated backup logs and all backups finished OK. It took me a week to find out the cause of our problems, fortunately all is running smoothly again.