Manual backup is not working

I am trying to backup some courses in my moodle environment, but I get the following error:

This page contains the following errors:

Below is a rendering of the page up to the first error.

The first error I got was about the execution time. I then increased the parameter max_execution_time to 180 seconds (the original value is 30 seconds). After that, I started getting the message above, when trying to backup any course. I am at a complete loss as to how to solve this problem. The line number and column varies according to the course.

My environment:

Ubuntu 11.04
PHP:  5.3.5-1ubuntu7.2
MySQL: mysql-server-5.1 / 5.1.54-1ubuntu4

Thanks for your help,

Rubens