If you suspect in your memory settings, just add following lines into your .htaccess:
# Increase the memory limit to 1G
php_value memory_limit 1G
# Define the script timeout after 1 hour
php_value max_execution_time 3600
I suspect on your Magento and code, so next time you run export, open the developer's console (in your browser), and check the ajax call's response, maybe it gives you more information (maybe it's error 500).
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