I have faced this error twice, I sloved it the first time by “Enabling the 32-bit Applications” on the application pool level of the workflow.
But in another time the installation was on Windows 2008 R2, and this made things harder. Enabling the 32-bit Application did not solve the issue. This actually happened because of the conflict in this Windows between the .NET framework 2.0 and .NET framework 4.0. It also might be caused of an authentication issue, so you better check that all one by one to get this validation problem resolved.
To solve the issue, I basically have gone through these steps: (of course I followed those steps after installing the workflow and I did not uninstall it)
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Deleted the application from the IIS (the selected node in the picture below).
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Added a “Virtual Directory” to the same site and named the new virtual directory with the same name of the deleted application.
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In the physical path of the virtual directory, I pointed to the installed worklfow folder (most probably will be in “C:Program FilesMicrosoft Dynamics AX50Workflow“). This folder contains the web services of AX workflow.
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Then, I converted the new “Virtual Directory” to “Application” by right-clicking that directory and clicking “Convert to Application”.
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I went to permissions of that application, by clicking (selecting) the “Applicaiton” you have just converted and double clicking ”Authentication” and I insured that “ASP.NET Imporsenation” and “Windows Authentication” are enabled and any other things are disabled.
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(Optional) You might disable the Kernel-mode authentication for the Windows Authentication by clicking the “Advanced settings” for the Windows Authenticaion. You cannot do that if if you have a kerberos authentication configured in your environment, if not.. then you better disable it to improve your environment performance.
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I then went to the “Handler Mapping” of the Workflow Application. I selected “WebServiceHandlerFactory-ISAPI-2.0″ and then clicked “Edit”. I insured that the “Executable” path is pointing to the .NET framework version 2.0 32-bit. (Note that this Windows contains on two versions of framework handlers and in two different folders inside the “C:WindowsMicrosoft.NET” folder.) In our case, you always have to point to the “aspnet_isapi.dll” file in the “Framework” folder not “Framework64″.
By this, you will be able to browse anyone of the *.asmx files in the Workflow Application. Also you might validate your workflow configuration.
Hopefully it has been resolved to you as well. 
