I wonder if this is activating a theme that had two sidebars, adding the Page Widget to what would be the 2nd sidebar, then changing to a theme with just one sidebar?
If so, try changing back to a two sidebar theme, removing the Pages widget, then back to the single sidebar theme.
Also guess you could delete the ‘widget_xxxxx’ option names in wp_options or set their values to a:0:{}
If that is the issue, please report back here. Thanks.