Can we access (read, create, update and delete) custom settings from a managed package via Apex code? The person manipulating custom settings will be using Salesforce Platform license, not Salesforce license. We can make it work with Salesforce license. In our case, we get the permission denied error.
I think your class must be written using 'with sharing'. If class is not written 'with sharing' then it will run in system context and will have full access to do curd operation on Custom Setting.