Diagnosis
The trace can only be activated for one user or group of users on eachapplication server. The enqueue trace has already been activated for thespecified user.

Procedure
Wait until the other user deactivates the trace. You can then activatethe trace. You can also activate the trace for all users, or for userswhose names match a pattern. When doing so, specify the pattern in theusual format.
You can activate the trace for multiple users by choosing the "ActivateTrace with Filter" button.