permlinks bug?

You are only using /%postname%?
If you do this, it treats /wp-admin/ as a popstname and attempts to open index.php with a post titled “WP Admin”. In fact, any folder on your site will get treated like this.
If you use this permalink structure you also cannot have the same postname on different days I believe.
From my understanding it is recommended to use something before /%postname%, such as the recommended permalink structure in the options, which uses /archives/ in addition to the date of a post.

Glad it helped!