I’m sorry you’re hitting up against difficulties, Don.
Unfortunately when you try to page to the next or previous month an out-of-memory error is taking place, causing the breakdown.
Fatal error: Allowed memory size of 41943040 bytes exhausted (tried to allocate 32 bytes) in /.../wp-includes/formatting.php on line 209
There are a few possibilities here and one is simply that you need to increase the memory available to WordPress from 42M to something higher (see this helpful article by our friends at WooThemes).
Alternatively, it could be that memory is being used up by a plugin or customization gone wild – so if you increase the memory limit and continue to hit the same problem, it may be worth running through our complete troubleshooting steps (including the steps to test if your theme is the culprit).
Let me know how you get on!