How exactly did you update the contacts to be marketing contacts (via bulk editing, workflow etc)? Can you share a screenshot of the error please? How exactly did you re-enroll the contacts?
Generally, I'd recommend placing a workflow action right before the marketing email send - as HubSpot has a workflow action "Set marketing contact".
Best regards
Karsten Köhler HubSpot Freelancer | RevOps & CRM Consultant | Community Hall of Famer
The issue was that the contacts I uploaded trough import, were not set as marketing contacts before enrolling them to the workflow. Thus they were not able to receive the emails. When I realised, I made them marketing contacts but the first 3 emails of the workflow were never sent.