I am getting the same error in 4.8.1 when passing parameters in Xmod. I see that you are also passing parameters. I have no problem with exactly the same code in 4.6.2.

On a hosted domain I get the same messages as you describe. When I ran it locally I got the following error, I suspect they are connected. Do you have a resolution yet? If not can anyone else help please?

 The HTTP verb POST used to access path '/DotNetNuke Website3/Template/tabid/55/Default.aspx;Title=title' is not allowed.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Web.HttpException: The HTTP verb POST used to access path '/DotNetNuke Website3/Template/tabid/55/Default.aspx;Title=title' is not allowed.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[HttpException (0x80004005): The HTTP verb POST used to access path '/DotNetNuke Website3/Template/tabid/55/Default.aspx;Title=title' is not allowed.]
   System.Web.DefaultHttpHandler.BeginProcessRequest(HttpContext context, AsyncCallback callback, Object state) +3439737
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +3391004
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +161