As Vladimir has already mentioned this is most likely caused by additional thread (non ee managed thread) started to completeWorkItem after task was completed. As WebSphere does not allow to access UserTransaction from a non managed thread thus you run into that issue. When using EJB I would recommend to use on of the following strategies for task service:
- use local task service that will participate in the same transaction as runtime engine and no additional threads will be created
- use JMS as message transport between runtime engine and task service using EJB (stateless to send messages and MDB to receive and process messages)
HTH