Magento: Manage Categories and 500 Internal Server Error

One of unique cases occur in Admin panel. But if you know the ins and outs of Magento and a little help from error log. The problem is easy to solved.

What do I find after debugging?

My life will not be easier without error log. Most hosting providers give you an access to error log.

Back to the case. ModSecurity iframe protection is blocking any embedded iframe in Magento. For your information, Magento embed an iframe to display category tree in Manage Categories.

My solution is to contact hosting provider. Ask them to disable iframe protection for your store.

Okay then, problem solved.