Following fix may help fixing the issue as there could be several other reasons. Please check the following if you are facing this issue.
One of the issues could be that you have an Organization Wide Default set as Private for Lead, Contact, Account and Campaign objects.
In order to fix this, you will have to create public group by adding “MassMailer Site Guest User”
Then you have to create Sharing Rule for the Objects which are Private
Sharing Settings--> Lead Sharing Rules Related list--> New
Label Name: MassMailer
Select which records to be shared : All Internal User
Select the users to share with : MassMailer
Once If It is saved then we can see like below
This should resolve the issue. If not, we can certainly look into your Salesforce Org and help. Please contact our support team for further assistance.