Exchange 2010 OWA "You need to choose a recipient before this message can be sent" error

Hi all, 
In our company we use Exchange Server 2010 SP1 with 4 servers (2 of them CAS Array with HUB role and 2 of them Mailbox DAG).  There is a problem as with a mailbox account: 

User want to send a mail to approximately 450 recipients as BCC from OWA, but he recieves"You need to choose a recipient before this message can be sent" and "an error has occurred and your request could not be completed. Problem persists, this HTTP contact your help desk with status code: 500 "errors. 

The error report contains 

Exception 
Exception type: System.Web.HttpException 
Exception message: Request timed out.  

If the user sends a mail to one recipient in OWA, it is sent with no problem.

If the user sends the same mail to same recipients in Outlook 2010, it is sent with no problem.

I applied the last Windows Updates, I moved the mailbox to different mailbox database, I rebooted the CAS&HUB servers but nothing changed.