Dear HegMS
Stabbing in the dark...sometimes the zipping process of the backup can run out of memory. If you are not using a resident zip program then I suggest that you use one if you can. In the detailed settings of your moodle there is a place to set a path to a zip-ping program in your server. On my university LAMP (or BSD AMP) it is
/usr/local/bin/zip
If you do not set a path to the zipper then php routines are used to zip the backup files, and when coures are large, these use a lot of memory, so that sometimes it can time out part of the way through the process.
The above has worked for some. But the problem may be elsewhere.
Another method of backing up the database is to use mysql (or the database that you are using ) or the database admin tool, mysql admin, which is available as an optional download to dump the database tables. I have been recommended to use 'the create dump tables' option when taking a backup of my database.
There are also a couple of routines on the forums for dumping zipping and sending the database to the administrator via email. I use one, but it does not seem to respond to the cron call that I am using. I can get it to send me the database if I ping the routine myself.
Restoring mammoth databases is a bit more difficult because again they often time out. If you paste the database in chunks it works.
Tim