JIRA hangs when assigning task to in progress status in the task board.

I've had a couple of instances where I would create an issue (a task in the planning board in the latest case). Then I switch to the task board and drag and drop the issue from open to in progress swim lane. After the drop event the board spins continuously and all JIRA pages become unresponsive. It seems to create some deadlock on the server side and requests from even new browsers are unresponsive.

I see the following in the logs prior to this occurrence:
ERROR ... /secure/GHAddIssue.jspa [velocity] RHS of #set statement is null. Context will not be modified. templates/greenhopper/jira/issue/fields/ranking-field.vm [line 12, column 1]