It was working all fine, until one day I started getting the error below when deploying the workflow using Visual Studio:

Error occurred in deployment step ‘Activate Features’: Unable to locate the workflow’s association data. To restore the association data to the workflow, restart the workflow settings wizard by selecting the workflow node in Solution Explorer and then clicking the ellipsis button (…) on one of the properties in the Properties window that has an ellipsis button.

Not sure exactly what’s going on, but selecting the workflow SPI in the Solution Explorer, then in the Properties pane, set Auto Associate to False fixes the issue.