The cause for this behaviour might be, that you have selected the main mail list via the "Tab" key gadget activation feature of MUI. MUI has a so-called CycleChain feature which allows to cycle through all existing gadgets in a window via the Tab key. In such a case, the main mail list of YAM can be activated via a numerous amount of Tab-key uses.

And as the NList-based main mail list changes it behaviour when it is directly activated via the CycleChain, this is the reason why the scrollbar scrolls when you use the up/down arrows. If an NList class is activated in such a way it will scroll down instead of changing the selection.

However, there is an easy way of solving such a situation. You just have to press "Ctrl+Tab" to signal MUI to unselect all currently active gadget and as such you will find yourself back being able to change the selection in the mail list via the Up/Down arrow keys.