When viewing the calendar page in List mode, there is a print button on the toolbar. When clicked it tries to redirect the same URL and attempt to add a print instruction to the URL querystring. This starts with the &-sign. This will work for the default permalink structure, in which there is always a querystring (except for home page), but fails with a status 404 for any variant of pretty permalinks.
If the is a querystring, start the tail with &
Otherwise, start the tail with ?