wp cron only runs if logged into job page

I have to use alternate cron for my server, so I have a scheduled task to run wp-cron.php which activates the job correctly, automatically. However it seems to pause at creating archive until I physically log into wordpress and access the job page (8.02 is the time I logged into wordpress, where the process then continued):

[INFO] BackWPup 3.2.0-beta1; A project of Inpsyde GmbH
[INFO] BackWPup job: Fullbackup
[INFO] Cron: 0 0 * * 1; Next: Mon, 20 Jul 2015 @ 00:00
[INFO] Logfile is: backwpup_log_0d38b4_2015-07-13_01-15-11.html
[INFO] Backup file is: BACKUP_2015-07-13.tar.gz
[13-Jul-2015 01:15:12] 1. Try to backup database …
[13-Jul-2015 01:15:12] Connected to database db01 on localhost
[13-Jul-2015 01:15:15] Added database dump “db01.sql” with 583.82 kB to backup file list
[13-Jul-2015 01:15:15] Database backup done!
[13-Jul-2015 02:15:03] 1. Trying to make a list of folders to back up …
[13-Jul-2015 02:15:06] 374 folders to backup.
[13-Jul-2015 02:15:06] 1. Trying to generate a manifest file …
[13-Jul-2015 02:15:06] Added manifest.json file with 5.54 kB to backup file list.
[13-Jul-2015 02:15:06] 1. Trying to create backup archive …
[13-Jul-2015 02:15:06] Compressing files as TarGz. Please be patient, this may take a moment.
[13-Jul-2015 08:02:41] Backup archive created.
[13-Jul-2015 08:02:42] Archive size is 15.76 MB.
[13-Jul-2015 08:02:42] 2829 Files with 42.10 MB in Archive.
[13-Jul-2015 08:02:42] 1. Try to send backup file to Dropbox …
[13-Jul-2015 08:02:43] Authenticated with Dropbox of user: ***
[13-Jul-2015 08:02:43] Uploading to Dropbox …
[13-Jul-2015 08:03:02] Backup transferred to https://api-content.dropbox.com/1/files/sandbox/***
[13-Jul-2015 08:03:03] Job done in 24472 seconds.

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