Thanks,
you saved my day! I used the wrong constants (or didn't know that there is no constant value for the second case).
A short note for readers of this thread: Return codes are not consistent for the two mentioned cases.
GetSetting(SETTING_TYPE_UI, UI_SETTING_SHOW_ACTION_BAR)
0 = Don't Show
2 = Automatic
1 = Always Show
GetSetting(SETTING_TYPE_ACTIVE_COMBAT_TIP, 0)
0 = Don't Show
1 = Automatic
2 = Always Show