I have opened the add-in button in my office online server but when I push it, there shows an error "Sorry, there is an problem" in Chinese.
I typed the code to open the add-in button:
Set-OfficeWebAppsFarm -OfficeAddinEnabled:$true
and have set the wopi server "AllowExternalMarketplace" to true.
But it still shows the error.
I have fixed the problem by update the OOS to the latest version. Hope this answer can help other developers.