Often by default arthemia will throw a 404 error when you click next page (from the main page). Most of the time this happens only when you get to the last page of posts that should exist.
Here is how you fix it:
- Go into you WordPress backend/dashboard
- Go to Appearance -> Editor
- Hit Main Index Template (on the right side)
- Find the section where it says:
query_posts("cat= - The end of this line will have something like “posts_per_page=5”
- Remember your number of posts per page
- Hit the settings button on the left and go into the Reading section
- Set “Blog pages show at most” to that number that you just found (5, in my case)