1.8 is end of life, and support for it ended 4 years ago. you really need to upgrade.
With the UI changes over the years I do not recall where you would want to go to check settings.
Things to check:
Department settings to see if new autoresponse are disabled.
Email settings to see if the autoresponse is enabled.
the Email address to see its settings to re-enter the password to see if that helps.
If it isn't one of those things, then I do not think that anyone here will be able to help you.