(In reply to comment #2)
> >> Are you absolutely sure that akonadi:?collection=262 contains items?
> 
> how to know this ? is there any tool to explore the akonadi db ?

There is a tool called akonadiconsole. Typically is packaged separately because
only developers need it. I don't remember whether it shows individual items.

There may be another check:

syncevolution --export - @default calendar | grep -e BEGIN:VEVENT -e
BEGIN:VTODO

If you see BEGIN:VTODO, then the tasks in the collection are impossible to tell
apart from events by their mime type, and therefore the SyncEvolution code
needs to be changed.

> kde calendar and kde tasks have the same "262" collection number as you can
> see above (see syncevolution --print-databases result) 

Yes, that's why it is important that only VTODO items are used when acting as
task source - the engine can only one or the other, but not both mixed in the
same data source.

> >> Do you know how to use gdb or how to recompile SyncEvolution?
> 
> - gdb : i hardly know it.
> 
> - compile
> 
> i only know to type ./configure , make , su -c "make install"
> if there is a pb i am lost

Let's try the check above first.