in some setups, UI2 with Alpha blending literally creates a situation where the individual component windows are NOT overlapping correctly at all. I do not know how to fix this, except to completely reinstall...and it only happens on the core systems, not with any external media directors.
the simplest solution for now, is to change your settings to just UI2 with Masking (The middle option), and your videos etc will play correctly.
not so simplest? try reinstalling to see if that fixes it.