Getting Error in new request set

Dear Experts,

On production we are creating a new request set which is containing with different reports. Once we submit a request   then some reports are complete with following error.

We also practice when we separately submit a single request of that particular report which was complete with error earlier separately it is completed successfully.

Or when we submit again this same request set then again some reports are complete with error but this time reports are different.

What is the cause of this rare error which is found in different reports?

Kindly assist us any workable solution which is rid off this problem.