I did some more tests. It appears that it works well in Google Chrome, but not in Safari for mac or the mobile Android browser.
It works fairly well for actually form pages, but installed in the widget the code does its weird redirect. The form itself reloads with the error message underneath the field, but the widget does not have this.
Offhand, my thought is that it is possible that, in the widget, the error message does not increase the widget size, in which case it may very well be there, but simply being covered up by the site sponsors widget below.