Errors only on scheduled backups

After upgrading to WP 3.5 I get errors in my scheduled backups. Manual backups work fine, however. I tried deleting and creating a new backup job, with the same results.
Here’s my latest backup log:

2012/12/21 04:41.05: [INFO]: BackWPup version 2.1.17; WordPress version 3.5; Copyright © 2012 Inpsyde GmbH Author: Daniel Hüsken
2012/12/21 04:41.05: [INFO]: BackWPup comes with ABSOLUTELY NO WARRANTY. This is free software, and you are welcome to redistribute it under certain conditions.
2012/12/21 04:41.05: [INFO]: BackWPup job: 1. Backup; DB+FILE+OPTIMIZE+CHECK
2012/12/21 04:41.05: [INFO]: BackWPup cron: 0 3 * * *; Sat, 22 Dec 2012 @ 03:00
2012/12/21 04:41.05: [INFO]: BackWPup job started by cron
2012/12/21 04:41.05: [INFO]: PHP ver.: 5.2.17; cgi-fcgi; Linux
2012/12/21 04:41.05: [INFO]: MySQL ver.: 5.0.96-community
2012/12/21 04:41.05: [INFO]: curl ver.: 7.24.0; OpenSSL/0.9.8b
2012/12/21 04:41.05: [INFO]: Temp folder is: /home/macha/public_html/wp-content/plugins/backwpup/tmp/
2012/12/21 04:41.05: [INFO]: Backup file is: /home/macha/public_html/wp-content/plugins/backwpup/tmp/backwpup_1_2012-12-21_04-41-05.zip
2012/12/21 04:41.05: 1. try for database dump...
2012/12/21 04:41.05: Dump database table "wp_commentmeta"
2012/12/21 04:41.05: Dump database table "wp_comments"
2012/12/21 04:41.05: Dump database table "wp_links"
2012/12/21 04:41.05: Dump database table "wp_options"
2012/12/21 04:41.05: Dump database table "wp_postmeta"
2012/12/21 04:41.05: Dump database table "wp_posts"
2012/12/21 04:41.05: Dump database table "wp_term_relationships"
2012/12/21 04:41.05: Dump database table "wp_term_taxonomy"
2012/12/21 04:41.05: Dump database table "wp_terms"
2012/12/21 04:41.05: Dump database table "wp_usermeta"
2012/12/21 04:41.05: Dump database table "wp_users"
2012/12/21 04:41.05: Database dump done!
2012/12/21 04:41.05: Add database dump "macha_db.sql" with 588.48 KB to backup file list
2012/12/21 04:41.05: 1. try for make list of files to backup....
2012/12/21 04:41.06: 2666 files with 38.57 MB to backup
2012/12/21 04:41.06: 1. try to create backup zip (PclZip) archive...
2012/12/21 04:41.11: [ERROR] Zip archive create error: PCLZIP_ERR_MISSING_FILE (-4) : File '/home/macha/public_html/wp-content/w3tc/dbcache/0/8/7/087d4dfefd91a7ae2d8533165c9e7e4b' does not exist
2012/12/21 04:41.11: Archive size is 16.08 MB
2012/12/21 04:41.11:...etc

Or could it be a problem with the caching plugin I use – in that case I don’t understand why a manual backup works fine.
I appreciate any help you can give.
/Peeter

http://wordpress.org/extend/plugins/backwpup/