Although user role “author” has the upload_files capability, the acf form doesn’t accept their files and tells them to try it again later. Files are not uploaded, it only appears so, but then the error is thrown.
// UPDATE //
this issue occurs, when hiding WP Admin dashboard from non-admins! It’s not the field itself that has issues, but as soon as I remove the hide-backend feature, the image upload works fine.
The following code hides the backend and frontend form still works fine: