EDIUS supports either straight (unmatted) or premultiplied (matted) alpha channel as designed. If you select premultiplied, you need to select black background color. If another background color is selected, the issue may occur.
In an application, select supported type when you create a file that will be imported to EDIUS. When you use premultiplied (matted), ensure that black background is selected in the application settings

Operation Example (Adobe After Effects)

Below is operation example to create correct file that has premultipled

1. In Output Module Settings, select Premultiplied (Matted)

2. In Composition Settings, select Black background color