Hi @Maxime BERNARD ,
Based on my research and testing, please follow these steps to fix this error:
1.Open "Central Administration".
2.Click "Application Management" in the quick launch bar.
3.Click on "Manage Service Applications" available under the Service Applications section.
4.Check whether the App Management Service application is created, if not then create the App Management Service application (as the screenshot shows).
5.After created, click on "Application Management" in the quick launch bar
6.Click on "Configure service application associations" available under the Service Applications section.
7.Check whether the App Management Service proxy is added to the default proxy group, if not then add it to the default proxy group.
8.Click on "Application Management" in the quick launch bar.
9.Click on "Manage services on server" available under the Service Applications section.
10.Check whether the App Management Service is started, if not then start the service.(As the screenshot shows)
11.Finally you can go to _layouts/15/appregnew.aspx
and re-create. By the way, if you click create and get an error :An unexpected error has occurred. You can wait a few hours and try again with a different browser.
Hope it can help you. Thanks for your understanding and support.
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