GuiXT seems not to be able to Open a file for reading  when opening a new session with "/O...". What could be the reason? The script works fine if it is not started with a "/O..." command.

When an InputScript is started in /O... mode, the processing starts with the first Screen command in the InputScript. Any script lines before the first Screen command (with the exception of Parameter) are not processed. In normal mode they are processed when you are on the screen in which the InputScript is started.