I have pruned all my site of unused plugins, themes, unnecessary language files, etc to get to the minimum currently in use.
Theses are all the plugins installed;
wp-content/plugins/1-flash-gallery
wp-content/plugins/add-multiple-users
wp-content/plugins/akismet
wp-content/plugins/all-in-one-event-calendar
wp-content/plugins/all-in-one-seo-pack
wp-content/plugins/bbpress
wp-content/plugins/contact-form-7
wp-content/plugins/cyan-backup
wp-content/plugins/editor-lock-by-wisdmlabs
wp-content/plugins/flash-gallery
wp-content/plugins/front-end-editor
wp-content/plugins/gallery-slider
wp-content/plugins/google-sitemap-generator
wp-content/plugins/gtranslate
wp-content/plugins/gtw
wp-content/plugins/hide-admin-bar-from-non-admins
wp-content/plugins/m-vslider
wp-content/plugins/memberfindme
wp-content/plugins/paid-memberships-pro
wp-content/plugins/participants-database
wp-content/plugins/pmpro-check-levels-master
wp-content/plugins/pmpro-extra-expiration-warning-emails-master
wp-content/plugins/pmpro-international-addresses
wp-content/plugins/pmpro-register-helper
wp-content/plugins/pmpro-user-pages
wp-content/plugins/really-simple-captcha
wp-content/plugins/s2member-files
wp-content/plugins/s2member-logs
wp-content/plugins/si-contact-form
wp-content/plugins/solid-code-theme-editor
wp-content/plugins/theme-my-login
wp-content/plugins/wordpress-importer
wp-content/plugins/wp-multibyte-patch
wp-content/plugins/wp-native-dashboard
wp-content/plugins/wp-user-frontend
wp-content/plugins/wptouch
I have selectively excluded / included each one of them whilst trying the backup each time.
I have got the list down to just 3 exclusions;
wp-content/plugins/all-in-one-event-calendar/
wp-content/plugins/front-end-editor/
wp-content/plugins/1-flash-gallery/
If I include any one of those, the backup fails and no zip file is completed.
At the moment, with those excluded, the backup is kind of on the fence, meaning that it will run and complete the zip file but it throws an internal server error and backup failed message. I can download and unzip the file ok and it send the backup ok email.
The backup.active file is still there and the status.log shows this;
96
Deleting /homepages/35/d30352212/htdocs/winton/backup/winton.20140313.164706/winton/wp-includes/css/jquery-ui-dialog.css…
active
/homepages/35/d30352212/htdocs/winton/backup/winton.20140313.164706.zip
2014-03-13 16:47:220
Each one of those 3 plugins excluded has more files than any of the others.
Maybe its the number of files causing the problem ?