Adding a .jsp containing the following code will work in 2.9, but produces an exception in 2.10 when a parameter such as os_destination is supplied:

<%
response.sendRedirect("/dashboard.action");
%>

Example URL: http://localhost:8080/confluence/login2.jsp?os_destination=%2Fdashboard.action

Typical exception:

org.apache.jasper.JasperException: java.lang.IllegalStateException
at org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:541)

caused by: java.lang.IllegalStateException
at org.apache.catalina.connector.ResponseFacade.sendRedirect(ResponseFacade.java:435)

This was encountered by a customer using external authentication, but has been replicated locally without anything external involved.