This only applies if you have the 1.1 and 2.0 framework installed and your workflow "addin" is built in .NET 1.1.
Create a new file called CRMWorkflowService.exe.config and copy xml below. Copy file to the same directory as that workflow exe
<requiredRuntime imageVersion="v1.1.4322" version="v1.1.4322" />
<supportedRuntime version="v1.1.4322" />