There is an implementation restriction on OpenActivities.

There is an implementation restriction on OpenActivities. When you query this relationship, security evaluation is implemented for users who don’t have administrator permissions, and you must use a specific sort order: ActivityDate ASC, LastModifiedDate DESC. can any one please help on this. 

I"ve talked about these implementation restrictions before.  Look here: http://community.skuid.com/skuid/topics/implementation-restriction-on-openactivities .  This is not somthing you are going to be able to get around with Skuid. 

Thanks for responding.

I read this link before posting my query. 

In my page two models and one PopUP(this is used to create new task)
1) Object and Object History.
I granted view all and modify all to user for particular object.

I tried by removing the PoPUP also but still i am facing. How to resolve this.

If you are creating a new task - do not use the OpenActivities sObject.  Use the task sObject.  You should be able to query other sObjects toward prepopulating data in your task, but if its OpenActivities - you can only query a single record.