Scroll bars have an issue in 3.5.5 that got fixed in 3.5.6, so try it after tomorrow's update.
Just additional info:
I found that if the dialogue (UIPanel) was not initially deactivated (ALT+SHIFT+A shortcut) at design time,
this bug does not appers. I.e. if a leave dialog active and hide it progrtammatically, UIScrollView + UIScrollBar behaves ok.