SharePoint Designer 2007 “failed to load the workflow”

To resolve the error “failed to load the workflow” when opening a declarative workflow in Microsoft SharePoint Designer 2007, please follow the steps provided below:

1. Close Microsoft SharePoint Designer 2007
2. If using Microsoft Windows XP, navigate to the directory:
   C:\Documents and Settings\[USER NAME]\Application Data\Microsoft\SharePoint Designer\ProxyAssemblyCache\
   If using Microsoft Windows Vista, navigate to the directory:
   C:\Users\[USER NAME]\AppData\Roaming\Microsoft\SharePoint Designer\ProxyAssemblyCache\
3. Delete all folders with the naming convention 12.x.x.xxxx
4. Start Microsoft SharePoint Designer 2007 and open the desired workflow

Please be aware that it may be necessary to perform these steps multiple times.

UPDATE (09/28/2009):
A possible cause of this behavior may be related to the use of custom activities within the workflow.  If the above solution does not resolve the issue, it may be necessary to redeploy/reinstall any custom activities that the workflow may reference.

[Scott Muller's SharePoint Blog]