>
>The debugger doesn't start at all. I'm using VFP 7.0 with SP1.

What happens if you change the assert command with

IF (NOT llAsertCondition) AND (Application.Startmode = 1)
   DEBUG
   SUSPEND
ENDIF

llAsertCondition stands for the condition you wish to assert.
This is not solution, however with it we can find out whether the problem is from the debugger or from the assert command.
Is your form modal? Check tools-options-debug [environment] combo box value - it should be Debug Frame or FoxPro frame - tell me what is your setting.

Regards