Irrespective of the stacktrace, which would only show java call stack, the control should be at the application level or the application server like weblogic/ Glassfish etc. The customer may be using one of this. There should be some place where the call gets into the java layer and that is where the synchronization is advised.
However, since there is no testcase, we have nothing much to analyze. So I have just created an FVB with the changes requested by the customer.
There are bundles for all supported platforms. You can choose any one of them.
I will need a confirmation from the customer. Without which I cannot proceed with integration. I understand that this would take some time as the frequency of the occurrence of the issue is not predictable. Hence I am moving the CR to afa (awaiting field action).