I uploaded again question2answer 1.5 beta 2 to my web server and uploaded all the plugins.
It was the wiki plugin. When I removed the wiki plugin from qa-plugin , the problem got resolved.
What hapened and why it happened , I still don't have any clue but I deleted every plugin that I had uploaded and it was the wiki plugin which when deleted, restored the problem faced by me.
Now I don't have problem in faq plugin too.