Accessing records from subgrid causes a permission error

We recently migrated our data to CRM Online and some users are facing a problem of not being able to access data from a specific Activity entity from the subgrid, or from the related entities link.

Those users have full permissions on both the entity shown in the subgrid and in the one where the subgrid is shown.

Those users can access all the records when going through the regular list (not through a subgrid).

Before the migration those users were perfectly able to access the data from the subgrid.

The error message shown is: You do not have permission to access these records. Contact your Microsoft Dynamics CRM administrator.

We do not have any plugin running on the Retrieve step for any entity in the system.

We cannot access the trace log, since CRM is Online.

Any hint?