Are your text and continue button on a UI Canvas? If so, is its Sort Order less than 32766? The Dialogue System's fade canvases are on Sort Order 32766 or higher.
If that doesn't help, how are you doing the fade to white? Fade() sequencer command?
I sent you two emails. The first asked for reproduction steps. I think you can disregard that one. The second email explained that the white background is the NPC Subtitle Panel's Portrait Image. Since no portrait image is assigned to the NPC's actor in the dialogue database, it sets the Portrait Image to blank white.
A quick fix is to delete the NPC Subtitle Panel's Portrait Image GameObject if you're not using portrait images.
I also had some suggestions about UI anchors and scaling.
If you didn't get the email, please let me know. It had a lot of inline images, so it might not have gone through if your email account has a limit on the size of emails it can receive.