IBM Worklight: No SessionManager in calling Worklight adapter

IBM Worklight: No SessionManager in calling Worklight adapter

Sometimes when Worklight adapter is invoked, the following error "No
SessionManager" is encountered in server log. When the adapter is invoked
again, result is returned. I have checked that the session ID remains the
same.
May I know what is the possible reason to have this kind of error? Thanks!
Environment: Worklight 5.0.6.1
[2013-09-09 17:55:38] FWLSE0099E: An error occurred while invoking
procedure TestAdapter/getTestDataFWLSE0100E: parameters:{
"arr": [
]
}
No SessionManager
FWLSE0101E: Caused by: null