I found something similar when attempting to update my RPi3 from 7.90.003 to 7.90.004, either via the automatic update function or using the CLI wget command. The file being downloaded to the /storage/.update directory was a mere 175 bytes in length. Upon rebooting the RPi the same "missing System or Kernel" message was displayed and the update would not complete. I'm going to take a wild guess and say that there's a truncated file in the mirror to which my download was pointed.
I solved this by downloading the 7.90.004 file, LibreELEC-RPi2.arm-7.90.004.img.gz (your distro will be different for the Wetek system), unzipping the file and copying the resultant file to the /storage/.update folder and rebooting. The upgrade completed without further issue.