Data Synchronizer stop working without reason....

When we try to regioster a device we get, on iOs devices, "impossible de vérifier les données du comptes". Must be somethings like "Unable to check account data".
On default.pipeline1.mobility-AppInterface.log, we get error messages :

2012-08-14 14:32:44.588 DEBUG_VERBOSE [Device_Thread-18] [__init__:1314] [userID:] [eventID:] [objectID:] [WSRThread] Socket 35: Total: 15 ,Idle Threads: 15.
2012-08-14 14:32:44.612 INFO_VERBOSE [Device_Thread-18] [DeviceInterface:253] [userID:] [eventID:] [objectID:] [] Invalid request. Not an activesync device..

Any good idea?

Thanks in advance.