It CRASHES when i CTRL-click a item in the backpack
^waht he meant was "if this is not your normal assertions of inventory space"
whatever that means
CTLR+click might be the key to copy something (if your an Operator). That could be the reson for the crash. But nobody can really help without a crash-log, that you should send to SirSengir in that case.
Furthermore, I think the comment in french says that "it's normal when you don't have enough space in your inventory" when "asser" should say for "assez", but anyway, a crash is nothing intended and should therefore not be "normal"
The correct translation for that sentence is "This is normal if there is not enough room in your inventory", which is probably true. It seems that the backpack isn't compatible with the ctrl-click from invtweaks, so the result is it tries to put items in your inventory without checking if it is full first. Obviously minecraft doesn't bode well with trying to put two items into the same inventory slot, so it crashes.