I’ve had no trouble accessing the API documentation but now suddenly it shows a login window (see image). I tried login with Safari and then reopen the API documentation but it still shows the login window. And if I try to write my password it says the file is locked for editing.

I’ve had no trouble accessing the API documentation but now suddenly it shows a
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Are you using the latest version of Xcode? I too had this problem a while back but I thought they fixed it in version 4.2.1 (Build 4D502).
From the 4.2.1 changelog: