Why do all auto created urls in my multilingual site cause a Page not found?
Check your configuration at Configuration -&gt; Languages -&gt; detection and selection: the order specified there could be what explains the current behaviour in your site.

You'd have to experiment a bit with what the exact order in your case should be. In my experience, I often get these kind of issues to work by using an order like 'URL, Default, User', and not something like 'Default, URL, User'. It could well be that that is all that's needed here ... So try to experiment a bit with the various combinations in this order to see if it helps.

Note: Debugging is the art of taking away all possible causes of a problem. So sorry upfront if this answer turns out to not be the solution.