You don't have sufficient permissions. This operation can only be performed by a manager of the group

You don't have sufficient permissions. This operation can only be performed by a manager of the group.

Say what? I'm the Exchange admin.

I don't know why this happens, but there's a way to get around it.

C:\>Set-DistributionGroup -Identity "yourdistributiongroupname" -ManagedBy "username"

You don't have sufficient permissions. This operation can only be performed by a manager of the group.
    + CategoryInfo          : NotSpecified: (:) [Set-DistributionGroup], OperationRequiresGroupManagerException
    + FullyQualifiedErrorId : [Server=SR-xxxxx,RequestId=3aed658e-071f-4ddc-abd9-04cf5450b8b8,TimeStamp=18-9-2020 12:2
   3:03] [FailureCategory=Cmdlet-OperationRequiresGroupManagerException] 22D1312B,Microsoft.Exchange.Management.Recip
  ientTasks.SetDistributionGroup
    + PSComputerName        : sr-xxxxx.domain.lan

To solve it add -BypassSecurityGroupManagerCheck

C:\>Set-DistributionGroup -Identity "yourdistributiongroupname" -ManagedBy 'username' -BypassSecurityGroupManagerCheck

Now you can manage the group like the admin that you are.