I agree with Brenden that it seems to be content related, is there really not changed anything? If so look what is changed even if it seems farfetched and start you investigation there.
But we had two weeks almost the same error on an website that didn't changed for the last couple of months. We searched and searched and couldn't find anything. As last attempt we restarted the application (system -> restart application) and we don't why buy magically it worked. Strange but maybe it is worth the try.