Call with dialog: In an InputScript we use a "Call" statement that executes a function module. In this module we call the function "SYSTEM_ATTACH_GUI" and then process a user dialog. After upgrading our GuiXT version we now find that the called screen does not vanish and is blocked after returning from the call. Is this a bug in GuiXT?

As of GuiXT version 2011 Q3 1, the RFC connections are kept between calls. If the called function processes its own screens they are not removed by SAP GUI after returning from the called functions, since the RFC connection is not closed.
You can avoid this problem by executing the statement

ProcessingOption RfcDisconnect="On"

after the Call statement. In this case the connection will be closed and the screen will be removed by SAP GUI.