I think I maybe solved the issue? I mean, its def a workaround lol but it can help MacOS users in the meantime -- original post on Naocat's thread regarding a similar issue on Mac RE: avatar not loading.
This method bypasses the need to press ESC (which throws that -50 error) and allows MacOS users to directly upload their asset folder to the corresponding AppData folder the program defaults to using. GLHF!
Nyaphomet
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I've had the same issue with my Mac, running 14.3 Sonoma public release. Before you attempt this fix, be sure to open PNGTuber+ at least once on your Mac to create the relevant folder.
1. Navigate to your PNGTuber+ AppData folder
/Users/(username)/Library/Application Support/Godot/app_userdata/PNGTuberPlus
2. While inside your PNGTuberPlus folder, copy and paste your avatar asset folder into your PNGTuberPlus folder
3. Open PNGTuber+ as normal and your avatar should still not be available to view; that is normal
4. Click the +Sprite button and it should pull up your PNGTuberPlus folder from previous steps
Your asset folder should be added and you can use the program as normal.