Hi Ilya,
The currentUser context is only available in the transaction that is triggered by the user. That means, any EL expression that is executed by the job executor (i.e. after asynchronous continuation) and that uses currentUser fails. If you need to preserve that context, you could consider storing the current user in a process variable when the task is completed, for example by adding a task listener on the complete event.
Cheers,
Thorben