by AL Rachels.
Francois,
If I am not mistaken, it means your php max_execution_time is set for 30 seconds. Since you say there are hundreds of submissions, php is running out of time before it can complete making the zip file to download all submissions at one time.
If you use groups like I do, you can try to download one group at a time. If not using groups, you will need to change the php_max_execution_time in the php.ini file to a greater amount of seconds. (I am using 45 seconds but have a fairly fast cpu and never download more than 70 submissions at a time.) You will also need to restart the web server to put the change into effect.
Hope this helps,
AL