EasyIndex uses WordPress's permalinks table to make SEO friendly URLs work for indexes. For some reason we haven't been able to work out, WordPress's internal permalinks structure can sometimes get corrupted. Although it doesn't happen frequently, a quick Google search shows that it's not uncommon.
If the internal permalinks structure gets corrupted, the most common symptom for EasyIndex users is that indexes that were once working perfectly, suddenly start returning a "404 Not found" error.
Fortunately, the solution is quick and easy.
Go to the WordPress Admin menu, click on the "Permalinks" link under the "Settings" option. On the Permalinks Settings page, click the "Save Changes" button. Note that you don't actually have to change anything - the "Save" action triggers a rebuild of the permalinks table internally.