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

Procedure
Wait until the other user has deactivated the trace. You can thenactivate the trace. You can also activate the trace for all users, orfor users whose names match a pattern. When doing so, specify thepattern in the usual format.
You can activate the trace for multiple users by choosing the "ActivateTrace with Filter" button.