YouTube Private Messages will be removed July 9

As of July 9, 2018, you will no longer be able to send, receive, or read private messages in Creator Studio. All private messages will be deleted and disappear from Creator Studio on that day.

This only affects http://www.youtube.com/messages

You can use YouTube Contacts to chat privately
https://support.google.com/youtube/answer/6401182

Details in the YouTube Help Center
https://support.google.com/youtube/answer/57955

Announcement:
https://productforums.google.com/forum/#!category-topic/youtube/O0y5k6nH1os
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