I tried the new Addon at
http://addons.oscommerce.com/info/8292 using the new sage_pay_server.php file, but still experience the same issue, with the 5006 error code.
Being honest, I am currently waiting on client to finalise setting up of new account with a different payment processor.
I receive this error, using both 2.2 and a clean install of 2.3.1, and also with SEO URL's turned off in 2.2. It could be a setting on my server, or something else. but this has me beat!