CRM 4.0 Report Generation 401.1 Error

Hello Guys,

Currently we are using CRM 4.0 On-Premise.

When a user (say John, Who is having System Admin role in CRM), generate report in for particular entity from (20 - April -2016) to (31-may-2016), the report get generated absolutely.

But when he changed the from date (12-April-2016), report throws 401.1 Unauthorized Error. Where the logic is going wrong.

Please help me to resolve this issue. Thanks in advance.