[Milhouse]

I edited my original post while you answered my question. I would be glad if you would tell me if flashing a new test build would fix perhaps remaining errors on system partition as well?

No, upgrading using a tar file will not by itself fix any filesystem errors in either the System (FAT) or Storage (ext4) partitions.

However, during every boot the System and Storage partitions are checked for errors that will - if possible - be automatically corrected. This check is disabled if booting from network file systems, or if "nofsck" is added on the kernel command line.

Writing a new disk image to the SD card will "fix" any existing filesystem errors.