Exchange 2013 CU6 coexistence with Exchange 2007 RU13. ActiveSync don't work. HTTP Error 500

Hi all.

In our organization we plan to migrate from Exchange 2007 to Exchange 2013. Exchange 2007 updated to Update Rollup 13.

Exchange 2013 with CU5 worked normally with Exchange 2007 and users from Exchange 2007 can connect to your mailboxes successfully.

After updating Exchange 2013 to CU6 users from Exchange 2007 cannot connect to your mailboxes via ActiveSync. All other services work normally.

Testing Exchange ActiveSync connectivity from web site https://testconnectivity.microsoft.com return error:
Testing of the OPTIONS command failed.
An HTTP 500 response was returned from IIS7.

In IIS log file (on Exchange 2013 server) I see HTTP error 500.

In IIS log file (on Exchange 2007 server) I see this error:
2014-09-03 08:43:17 W3SVC1 <Exchange2007_IP> RPC_IN_DATA /rpc/rpcproxy.dll - 443 - <Exchange2013_IP> HttpProxy.ClientAccessServer2010Ping 500 0 0

When I try to synchronize mobile device
- mobile device return an error 85010014
- in Windows Application log on Exchange 2013 server I see event ID 4999:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
<System>
<Provider Name="MSExchange Common" />
<EventID Qualifiers="16388">4999</EventID>
<Level>2</Level>
<Task>1</Task>
<Keywords>0x80000000000000</Keywords>
<TimeCreated SystemTime="2014-09-03T08:15:56.000000000Z" />
<EventRecordID>463807</EventRecordID>
<Channel>Application</Channel>
<Computer><Exchange_2013_HostName></Computer>
<Security />
</System>
<EventData>
<Data>20832</Data>
<Data>E12</Data>
<Data>c-RTL-AMD64</Data>
<Data>15.00.0995.029</Data>
<Data>w3wp#MSExchangeSyncAppPool</Data>
<Data>M.Exchange.AirSyncHandler</Data>
<Data>M.E.A.Handler.LegacyUserAllowedToSync</Data>
<Data>System.InvalidCastException</Data>
<Data>9619</Data>
<Data>15.00.0995.026</Data>
<Data>False</Data>
<Data>False</Data>
<Data>w3wp#MSExchangeSyncAppPool</Data>
<Data />
</EventData>
</Event>

Pls, help to fix this.

 Michael.