Well, both of em have been granted access. :(

Nope. They are different

I logged in as one of the users who is having "Salesforce Platform" license. The user who created the reports have a different profile. The users with the same profile are able to see the report however for different profiles I am getting the error.

Just checked: They have the access- Below are the details: Read, Create, Edit, Delete, View All, Modify All

How do I check that? Here I am assuming the platform license can access the custom object