View workflow screen does not work (and parts of task details screen) in Alfresco 4.2a by default. Adding this to main function in webapps/share/WEB-INF/classes/alfresco/site-webscripts/org/alfresco/components/datadata-loader.js fixes the problem:
if (args.eventName != null)
{
dataLoader.options.eventName = args.eventName;
}
Actually the problem was introduced by ~5000 revision old changes in SVN. I wonder how many people are using workflows in community edition.