The problem is inside the way this plug-in redirects login pages. This reaction gives a small, but ugly fix.
If you leaf the option for the 'wich page' to redirect to empty, your browser thinks it loads the same page, and so uses its own cache. But we leaf this option empty in the widget to redirect to current page.
If you change this option from empty to a fixed page, the sidebar login works. But there by, leaving a important option, and main feature of this widget unable to use.
Fast ugly fix
Use another page (e.g.: /) to redirect to in the widget option.
Author fix suggestion
Make sure that when you redirect to the current page, you give the browser/header the option to force new page. Also you can add a variable in the GET header with a random number to make sure caching doesn't match the url, and the browser thinks it needs to get a new page.