I seem to have the same problem backing up large files.
As i can see large for my system means about 150M.
I have been following many forums for a week or so that i m trying to solve this problem.
From what i ve read i made some changes to php.ini
These are:
upload_max_filesize = 300M
max_execution_time = 3600
upload_max_filesize = 300M
My system is an Ubuntu 12.04 32bit,
PHP Version 5.3.10-1ubuntu3.9,
version 5.1.33 mysql, 2G RAM
Moodle 2.3.6 stable
The most common error when backing up a large file is "INTERNAL SERVER ERROR" and if i change debug to on it does not give me any clue in html screen.
After refreshing the page from "INTERNAL SERVER ERROR" gives me a "error/cannot_finish_ui_if_not_setting_ui"
Also automated backups after having "unfinished" in some courses gives an "error" in next try.
Any help will be appreciated,
Thank you