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A Reference File is Required Error - Already Attempted Relinking
AC Version: Version 1.77.3 (I think this was the most recent in the 'changelog' I have in my project)
Unity Version: 2121.3.39f1
Issue:
- I moved my AdventureCreator folder to a new one, titled 'Add-ons', in an attempt to clean up and organize my asset folder. It was a rookie mistake. Literally thousands of errors started appearing.
- I moved the folder back, but this didn't change the errors, even after refreshing.
- So, I scrambled online for a fix. I was quite upset and looking for anything that would help.
- I was advised to 'Reimport All' assets; I did this.
- When re-opening, the high number of errors was gone, but some remained, namely "No References " found. Please set one using the main Adventure Creator window."
- Easy, I thought—I'll just open Game Editor, Create a References file, and relink them.
- So, I did this. My asset files for each manager were still there, and AC allowed me to choose them. Yipee!
- But when I pressed Play - these assigned managers disappeared from the Game Editor window.
- Alas, I returned to Error - Missing References again - despite assigning them before Play.
One well-meaning attempt to tidy up my assets folder has resulted in a world of hurt and hours of painstaking work now broken. I am devastated!
I've made some video documentation of this issue: 
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Comments
Apologies for the hassle.
Try not to worry, though - your game's data isn't gone. The References file merely lets AC know which set of Managers to use. Without it, the Managers still exist - they're just not linked to the game.
This has been improved in more recent AC releases, but I suspect what's happening (since clicking "Create References file" doesn't update AC's Resources folder), is that it's being put somewhere else.
Try looking for a Resources folder in your Assets directory - the auto-created References file may have been put there. If so, move it back to /AdventureCreator/Resources.
It is possible to move AC's installation directory, as well. You should be safe to do so, provided you also update the "Install path" in AC's Editor preferences, from Unity's Project window. Given the earlier hassle, though, I'd strongly recommend backing up your project first.
I managed to resolve this, thanks Chris.