ASP to PHP

So I have I this problem. I have asp page that has a link to a PHP page, when I click on the link I get the PHP with no problems but if I click back to the asp page and click on the PHP page I get a HTTP 500 Internal server error. If I hit the refresh button I get the php page again. The PHP page is a form when the form is submitted after the fresh reload an error "the directory name is invalid". Can anyone help me? Thanks!