This is the most common siebel error code, developers ever face. There could be many reasons behind this error code.
1>Wrong using of CancelOperation and ContinueOperation in a custom business service. When we write any custom business service and we forget to use CancelOperation in an event which does not follow any other event, siebel throws this error.
2>When Siebel faces issues with user keys, this error message would come up.