Ok, so… It works, but please read under
@pinkishhue : i tried a short feeds, and it does not change. But, when trying your idea, i found a solution.
When you click on “syndicated sources”, there is directly a form “new source”. Using THIS field works (with any feed)
When you click this “add” button, either with no url, or with a password protected url, you arrive on an error page with either a “Alternative feeds” box or with a “Find feeds” box. There is a submit button called “check” and THIS one does not work.
Again : I was trying with a feed protected by username/password, so after adding the feed, you arrive on this error page where you can see 2 boxes : “Find feeds” and “Option 2: Unknown feed”. The first box does not works, the second (with the button “use this feed”) is OK.
This is a workaround, i’m not sure it should be declared as “solved”.