Similarly (but not), I just installed the MailChimp Widget plugin and the installation seemed successful. I entered my API key. But when I placed the widget, and opened the widget sreen, “select a mailing list” list was empty. And when I tried to use the widget on the front-end of the site, it provided the error message “there was a problem processing…”
I do not have the Archive plugin. Often, when I install widgets, if there is a problem, it’s often a permissions problem with javascript files; but that doesn’t seem to be the case this time.