Why am I receiving code 404 but the page is visible?

Why am I receiving code 404 but the page is visible? I’m having trouble crawling search engines because of this…

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View post on imgur.com

It’s not the themes or the plugins that cause problems…

If I am connected with wp-admin, the mentioned url works. A little while ago I tried incognito and also with logout and the page has the code 404.

The page I need help with: [log in to see the link]

 

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