This issue was raised from: https://support.atlassian.com/browse/GHS-581:

When trying to add a non-standard sub-task ("sub-improvement") to an non-standard task ("improvement") the system let me chose the "sub-improvement" at the listbox. But after the selection the "working" symbol continues to display infinitely.

The only way out is the "back" button.

The problem only happens in IE8 but does not happened in Firefox. In IE8, there is a Jscript error in file "gh-global-4.3-min.js", Line 1, Char 7743.