Updated 23/10/18: this issue has been fixed by Instagram. Please update your Instagram app to the latest version (v 68) in order to fix your manual posting!
Currently manual posting for iOS is incompatible with the Instagram update (v 67) which was released on Oct 16th 2018. When you click 'Post on Instagram' and are taken to the Instagram login screen to log in and post your media, you will receive the error 'Unable to open file. You must be logged in.'
Here is the workaround for this:
1. Tap the 'x' in the top right hand corner of the screen.
2. You'll be taken to your Instagram profile. Press the '+' button to create a new post as you would normally. (For Stories, tap the home button and then the Stories camera to add a new Story.)
3. As we download your scheduled media to your Camera Roll, you'll find the relevant scheduled media under 'All Photos'. Click to select it to post.
4. Your caption will still be copied to your clipboard, so all you need to do is hit 'Paste' when you get to the caption stage, as normal!
You can view the entire process in the gif below: