When publishing a workflow with a Flexi Task, the following error occurs:

Error saving from workflow export file.: System.InvalidOperationException: Content type for multi outcome tasks was not found.

This error means that the content type is missing, deactivating and re-activating Nintex features may help, otherwise follow these instructions:
- Create new site
- Activate Nintex Workflow and Forms
- Migrate lists and workflows to the new site