Du darfst als Permalinkstruktur nicht nur "/%postname%/" angeben.
Zitat:
Note on using only %postname%
If you use postname as the only element in your permalinks to create a structure such as myblog.com/post-title, the rewrite rules may make it impossible to access pages such as your stylesheet (which has a similar format) or the wp-admin folder. It's best to include some numeric data (e.g. the post ID or date) in the permalink to prevent this from happening.
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http://codex.wordpress.org/Permalink_Structure