Can you provide a URL where we can see this behaviour (the URL you provided when you created this thread seems to lead to a holding page). Thanks!
What I’d try in the first instance though is flushing the permalink/rewrite rules, just by visiting the Settings → Permalinks admin screen. However, it does rather sound like the problem boils down to a theme conflict, based on what you’re telling us. Can you provide any insight as to what your theme is and who wrote it?