Ben,

I'm handling it in the ConcurrencyException event of the BO. Never raised...

Local line is deleted in BO, not on server (I can see it by loading another instance of the same MaintenanceForm).

How could I debug that? where should I put a breakpoint?

If you have the SF source code built in debug, then put a break point in the DataLayer.vb file (in the MicroFour StrataFrame Business project in the Data folder) somewhere around line 1120 on the HandleConcurrencyException method.  This method will be called when the server returns 0 for the number of records affected by the delete.  You can see what it's doing by stepping through it.

ahem..

Ben, don't forget I'm an absolute beginner in .NET, so I don't know how to put this breakpoint in SF sourcecode in my project

Could you advise please, I'm lost now and here
Thanks

Michel