by Ken Task.
Check apache error logs. Could be that php variables for time to run, how much memory a script can use, etc. need to be increased. Also ... if you see 'server has gone away' ... that's a MySQL server setting ... max_packet_allowed ... is too small ... increase it. Config file for MySQL depends upon how hosted, OS, etc.
Check the moodledata/temp/backup/ directory. The 'log' files for failed backups can be seen there.
Any directory in moodledata/temp/backup/ that has a long name ... bunch of letters/numbers ... was a directory being used by Moodle to build a backup file. They are safe to remove manually and will not harm Moodle in any way.
'spirit of sharing', Ken