>> Are you absolutely sure that akonadi:?collection=262 contains items?

how to know this ? is there any tool to explore the akonadi db ?

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

>> 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

>> the "sub mime type" was changed from "text/calendar" to the more specific
>> "application/x-vnd.akonadi.calendar.todo"

yes in the list of the file types associated to apps there is
"application/x-vnd.akonadi.calendar.todo"

perhaps to avoid futur pb it would be easier not to hardcode the mime/type and
to have the ability to assign a value by the way of syncevolution cli tool.