When opening the configuration of a Job, the "Loading" overlay remains visible after the page has finished loading (see screenshots)
In Firefox there is a workaround as the overlay window is transparent, I can navigate with tab to the input field and can do the changes using keyboard only.
In Internet Explorer, the overlay window is grey, without any transparency, so it is not visible where to navigate to.

It seems as if this happens with a Maven2 project. There are multiple Free-Form (Ant) Projects and one Maven2 project. And only the Maven2 project shows this behavior.