eXtplorer - PHP-based File Manager

I'm using joomla 2.5 and extplorer 2.1.
I'm trying to archive the whole of my joomla folder (from the root). The error "An unknown Error occured" occurs at any time during the archiving process (ie: the file .tgz is created, the file .tgz.tmp is also present when archiving process managed to start (sometimes up to 69%)). Starting the same command again and again ends up with this error.

However, when I archive each sub-folder, either alone or grouped with others (ie: smaller amount) it's OK.

I did it with the box "upload" unchecked.

It's must be a timer delay or something.

Thanks

Anne-Marie