Intermittend login issue

With the login procedure I experience intermittent problems, i.e. it sometimes works, but sometimes stops with showing the following screen upon entering userid and password: https://snk.to/f-ct3qz22f (links to a site where the screen shot can be downloaded from)

The widget is active with the modal version and installed on a web page using the WordPress Twenty Ten theme.

I am having the problems with IE9/10 and Firefox 22.