http://forums.asp.net/t/1374211.aspx

i see your resolution posted on the above mentioned url and have pasted the
same below for the benefit of others. Hence i'm closing this request

"I have solved the problem by restarting sql reporting services or reboot
the computer.

And add the impersonate users to be the administrator group of the IIS
computer and sql report server (sql db).

add the impersonate users in the execution account of sql report server ."