Posts not displaying…

I have just upgraded to WP 2.6 and am having trouble getting older posts to display.
With WP set to display just one post, the most recent of past posts for any given month displays, but none of the others will display when I click the ‘next page’ link at the bottom of the page.
I get the following error message:
Page not found
The page you are looking for might have been removed, had its name changed, or is temporarily unavailable.
Please try the following:
If you typed the page address in the Address bar, make sure that it is spelled correctly.
– Click the Back button in your browser to try another link.
– Use a search engine like Google to look for information on the Internet.
HTTP 404 – File not found

When I set WP to show multiple posts (such as 3), the 3 most recent posts for that month will display, so the files are there. But displaying 3 posts at a time is a bit much, I feel.

I updated the WP software using the WordPress Automatic Upgrade plugin, and am currently in the ‘Fog’ theme found on the WP site.

Please advise, and thank you for your time.