Symptoms
Receiving error when deploying workflow (or when opening workflow in designer).
"Workflow contains an invalid form"
When deploying, at the Checking workflow properties.... task: this error: "The Workflow Settings contains an invalid form."
Diagnoses
The issue was caused by invalid form (old) was referenced in Workflow Settings (Start Activity).
Resolution
Editing Workflow Settings, and changed it to the correct form resolved the issue.