[Plugin: Efficient Related Posts] Won't activate, says server misconfigured

I tried activating and deactivating other plugins and they worked fine, but when trying to activate this one it takes a long time to “think” and then returns this:
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Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

http://wordpress.org/extend/plugins/efficient-related-posts/