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I've found that almost all of our 500 errors have been server related, such as a security issue being run into with the site. I usually work with the host to see if they can find something in their logs. The only other reason I've gotten a 500 error was when something in my php settings wasn't set high enough (like memory allocated, execution time for a script, etc).

Usually my host will have me run through the steps to recreate it (making sure they have my IP address) and they will watch for any related errors.