This issue is divided into 2 parts.
1) Server related 500 error
To fix this error you will need to increase memory and time limit in your PHP.
If it will not help, switch your PHP to suPHP.
2) Memory issue in old version of Magento 18.104.22.168
If you have old version of Magento e.g. 22.214.171.124. you need to apply this patch:
It will speed up export process.
We fixed 500 error issue on our hosting for our client recently.
So feel free to contact us if you have 500 error, we will help.