Hope you don't mind me adding my problems to this thread, but I have a similar problem in that the PP checkout keeps failing.
I am using the sandbox testing server; I have added the session echo to my failure page and I get:
["ECO_Process_Result"]=>
string(2254) "2013-01-22T12:03:48ZFailurecbaee1a559221Security errorSecurity header is not valid10002Error63.04181146"
I hope you can help me,
Regards,
Jarrad