hi
After Winter 14, If Community enabled, there will be two kind of OWD. External and Internal means what information should be visible to internal and external users. Also , there will be new setting named “Standard Report Visibility“. If it is checked user can see reports based on Standard report type even though they don’t have proper OWD and may expose sensitive information about internal user to external users (for example : Internal users role). If external user is getting an error whole running the report this setting may be one of the cause.
Read this.
Thanks