When a question is submitted on my site, a 500 error page appears. Please see the screen shot of the 500 error page.

1. If I click back button on browser, the blank form (question asking form) appears. But the question is actually submitted. I can check it by going to home page.

2. If I refresh the 500 error page, the question is submitted twice, and the 500 error page is still visible.

This error has recently come up. It never occured in last one year or so.