The error "The operation is not supported within an edit session." is displayed when the following conditions are met:
• A trace task is being run on a geometric network
• The geometric network is stored within a personal geodatabase
• The geometric network is being edited within an edit session
This is due to NIM087222.
Performing the trace task outside of an edit session eliminates the issue. This issue can also be solved so that the trace task can be run within an edit session by installing ArcGIS 10.2.
Note that even after installing ArcGIS 10.2; the error may still be observed if the following conditions are met:
• Geometric network is stored within a personal geodatabase
• There is at least one table join on a feature class with trace task results
• An edit session on a feature class with trace task results
The only workaround for this scenario is to remove one of the conditions.