WordPress: Updating Plugin BackWPup (3.4.0 -> 3.4.1) – bad update problem

An error occurred while updating BackWPup: Could not copy file. backwpup/vendor/Guzzle/Plugin/Cache/CachePlugin.php

The file exists and I have no idea, why this error message gets shown. Actually various files below backwpup/vendor/Guzzle/Plugin are concerned.

Update: That happened, because I exceeded my disk quota.