Hi tamas.antalkwikkfix.com, hi smartwebdev,
WordPress returns a status code 500 (Internal Server Error) by design if the comment form was not filled out correctly (like empty comment or invalid e-mail address). So this is not a problem.
Could you please send me a link where I can reproduce the problem?
Setting the z-index (default is 1000) to a higher value could be necessary (depending on your theme or customizings). Nevertheless, I would not recommend to set the z-index to value larger than 2147483647 (http://www.puidokas.com/max-z-index/).
Hope this helps...