Please ensure that no keyboard layout has been specified in System Settings > Input Devices > Keyboard > Layouts. As the default layout you use at the login screen works fine, you should disable this if it isn't already.

run "kcmshell4 keyboard" - you should be able to configure layouts there, the toggle and also advanced keyboard mappings.
try turnining off "configure layouts" and "configure keyboard options" globally. If they are, the setting might be applied somewhere else (eg. ~/.xprofile or similar)