Setting up a VM the other day and got the oddest error trying to create a Workflow in SharePoint Designer:
The list of workflow actions on the server references an assembly that does not exist.
Oddly enough, it was referring to the Microsoft.SharePoint DLL! So after a bunch of checking (bear in mind that this was not a special workflow, just a list workflow to send an email), the answer turned out to be incredibly simple:
It appears that if the system does not have enough memory, this error will occur - I was running a VM at 3GB; when I upped it for 4GB, viola! Works fine.