Error when I try to reactivate a system workflow

I deactivated a system workflow: 'Field Service - Generate Work Order for Agreement Booking Date' just to have a look over the steps in the workflow which came out of the box. Now when I  try to reactivate it, it throws this error:

The system could not log you on. This could be because your user record or the business unit you belong to has been disabled in Microsoft Dynamics 365. If you contact support, please provide the technical details.

However I am able to activate and deactivate my processes (workflows).

Any suggestion would be highly appreciated.