Changing the store view code = 404 page

Hi,
when I change the store view code from the standard “english” to “en” and “german” to “de” (which makes much more sense as standard, by the way) I get a 404 error. Of course, as usual, I have both flushed the cache and reindexed the data before re-loading the page.
Please check, I have already spent hours of research and try-and-error on this topic. You can still use the login infos I sent you last week.
Thanks,
Matthias