This is usually caused by the workflow ID of the step, pointing to the wrong group ID. Please raise a new case online and reference the title of this article.
For non-hosted customers:
To check if this is the case you'll need to sign into the database and run the 2 statements below.
select workflow_id from mo_workflow_steps where workflow_step_id = <<INSERT WORKFLOW STEP ID HERE>>
This will give you the workflow_ID associated with the step, and the below statement will tell you the group ID that is associated with this workflow ID.
select group_id from mo_workflows where workflow_id = <<INSERT WORKFLOW ID HERE>>
Now check and see if this group ID matches the ID of the group you are trying to start the workstep for. If not, run the update below, and this should fix the error.
update mo_workflows set group_id = <<Insert correct group ID here>> where workflow_id = <<INSERT WORKFLOW ID HERE (NOT WORKFLOW STEP ID)>>