850M isn't a Gig! So yeah ... not that large. 800 courses ... a lot to do.
And, normally, if one sees 800 in the .log file it finished with the actual backup build.
Then am gonna guess the backup script timed out at the *copy* stage where it copies the backup.mbz file there to desination and changes it's name according to preferences in backup options .... if there is a backup.mbz file there and it's valid ... that's the very last step.
Going back to GUI manual selecting modules to exclude will reduce the size of the course ... or should ... thus should also take less time to *copy* from build area to destination.
What are php settings for time for a script to run and memory a script can use?
Also check your task list for possible conflicts between what is probably a very long lasting autobackup process with any 'clean up' of temp areas.
How long does the autobackup process take? Something to check into for determining conflicts with cron/task. If it starts at midnight and doesn't finish before prime time for your site then ... also ... have to remember autobackup is really background ... server still has to contend with whatever else it's being asked to do.
'SoS', Ken