Hi,
From the log, it appears that UpdraftPlus is not able to process the files in your plugins directory. It appears that each run is being killed off early.
Please could you contact your hosts/server admin, and ask them to check the PHP error logs for the site? This should contain more information on the issue occurring.
You can also try reducing the backup archive split size. This will split the backup into smaller, more manageable files.
To do this, find the ‘Split archives every’ option in the ‘expert settings’ section of the UpdraftPlus Settings tab. Reduce this value to 50MB.
Best Wishes,
David N