Hi Michel,
I can see the problem. Your site's webserver is 32-bits (indicated by the i586 at the top of the log), which means that the maximum file that can be created is 2Gb. The uploads folder rarely compresses much (because images, videos + audio files are already normally compressed), so you have more dat+a than can be fitted into a single file on such a machine. This is an operating-system-level restriction - it can't be over-ridden by any program. Once a file hits 2Gb, nothing more can be written to it.
We plan to add code to spit up data into multiple archives, but we don't have an ETA for it. For now, your options are to move to a 64-bit webserver (most are), to prune some data from the uploads folder, or to look for a backup method that dispatches data file-by-file to the cloud, instead of making a single archive (I don't have any particular recommendations on that - I'm not familiar with other available solutions).
David