The 500 error on the homepage appears due to your Poll module under the following conditions:

1. I have a poll on the homepage.
2. I vote in that poll.
3. If I add a new poll, an error appears on the page.
4. Clearing the cache does nothing.
5. The error disappears only if I disable the poll.
6. If I open another browser before step 4, the homepage loads without error with the new poll.

In the attached image, the Poll can be seen in the top right corner.