It looks like a bug in the API. It looks like it filters on the creation date of the question, not the timeline events themselves. The question has a timestamp of 1596793060, and using a fromdate of 10 seconds earlier produces the full timeline:
An access_token is not enough; you need to specify a value for key as well. You can find it in the entry for your app here, this is how it looks like for one of mine:
Why is my app post deleted?
My guess is that an automated process did this. You can still view the post, just copy the URL from the 'Manage your applications' page. If it has a banner 'deleted by Community ♦' that's the case.
Should I just create a new post to get API access?
That's fine (it's not that we're flooded with new posts every day). Something ...
You are using explicit OAuth 2.0 flow.
When a user approves your app to access the requested scope, they are redirected to the redirect_uri with an authorization code. Your app should use the authorization code to request an access token. In this case, you are using the authorization code as the access token thus the error.
You can view more details on the ...
It is indeed possible to use custom URL schemes for authentication in an iOS app. Here are the steps I took:
Choose an arbitrary domain, like stackexchange.com, for the OAuth domain in your Stack Apps
(Optional) Make sure it has a Stack Apps Post if you want tokens with write access.
Check the checkboxes for these options:
Enable Client Side OAuth Flow
When using the API to add/edit posts or comments, you need to ensure the preview parameter is set to false, otherwise it's just a 'test' request which does nothing (but allows you to test if the response parsing on your side works correctly). Just like writing content on the site itself, the body needs to be in Markdown.
If you set preview to false, you'll ...
Is your app listed here? If so, you should edit its entry and paste the URL to this question in the field Stack Apps Post (optional). If you don't have any apps there, you need to create one first and add the question later.