my First Plugin: wp_login.php not working once plugin activated

I have created my first wordpress plugin, I am not that good in PHP and wordpress internals. My plugin is working fine as it supposed to be. No problem is setting the option and then works fine on blog.

Only Problem I am having is, once plugin is activated and I try to logout wp_login.php doesn’t do that.

Even when I try to login when plugin is activated, I can’t login.

In both cases when I hit submit button, I only see blank page.

Can someone please help, if I am missing something.

I tried in WP version 2.5.1 and 2.7, getting same problem in both.

Thanks for help