"App not registered" and kitten when opening
You are trying to use the widget in the legacy way (without any URL parameters (no access token)) when it is not enabled.
"App not registered" and kitten when opening the FotoWeb OAuth URL (/fotoweb/oauth2/authorize/...)
Your application is not registered, or you are using the wrong redirect URL in the authorization request.
Register your app using the correct redirect URI and use that redirect URI in the request:
Application registration using OAuth_2.0
If your app is registered with only one redirect URI, then you don't need to explicitly specify it in the authorization request. Not specifying it eliminates the problem of using the wrong one.
The redirect URL must be exactly as registered: Same path, same protocol (HTTPS vs. HTTP. I hope you are using HTTPS...). Query string parameters and fragments are not allowed.
The FotoWeb OAuth URL (/fotoweb/oauth2/authorize/...) requires a client ID. What is the client ID?
Your application is not registered, so there is no client ID, yet.
Register your app, and you get a client ID:
Application registration using OAuth2.0.
My integration broke when I updated to FR12
Your integration is using the widget in the legacy way when it is not enabled.
Either enable "Legacy selection widget" in the site settings, or update your integration to use the new (more secure) way: Using embeddable widgets
My integration still works after FR12 (with legacy enabled), but it looks ugly / I don't like it
We had to change the user experience to make it work with OAuth.
Update your integration to use the new way. It will give you more freedom to determine the user experience for user authorization. See Using embeddable widgets. We are happy to help you migrate!
Instead of the selection widget, I only see a blank white box.
You are embedding the selection widget in an iFrame in an app that uses HTTPS, but FotoWeb does not use HTTPS.
Configure FotoWeb to use HTTPS and open the selection widget using a HTTPS URL.
You are opening the FotoWeb OAuth URL (/fotoweb/oauth2/authorize/...) in an iFrame.
You can only open the FotoWeb OAuth URL in a top-level browser window or tab. Either open it in the current window, or in a new browser tab or window and use local storage to get hold of the access token. See the iFrames section in Using the widgets.
Instead of the selection widget, I see a white box with a Log in to FotoWeb button.
You are trying to use the widget in the legacy way (without any URL parameters (no access token)).
For information on how to use the widget, see Using embeddable widgets.
If you intend to use the legacy way, then this is the way it works. The user has to log in by clicking the button. If you don't like this, you can use the new way.