YAWL 2.1 has some serious performance issues : we have been using the YAWL engine V2.1 in a scenario of about 25 users accessing the system simultaneously. We had to restart the engine several times. After some analysis we found out that this was due to a problem in the YAWL engine. This problem has been fixed. For all who use YAWL 2.1 we recommend to upgrade to YAWL 2.3.