Excel report in BI Center returns error message

Hello 

Using Excel services, we get sporadically this error message

this is strange as refreshing the page may produce the report well

we tried the solution here without success (

http://social.technet.microsoft.com/Forums/sharepoint/en-US/ffde71c8-613d-408d-b885-6b0497251ebc/ewa-refresh-succeeds-but-fails-with-multiple-concurrent-requests)

does-any of you have ideas on what can happen and how to solve it?

thanks in advance

Jérome