Sandor but why both sides?
seems like a unlucky event then?
seen many things fails , and flutter can do a lot of damage.
even if one would not be glued correctly , both at the same time?

I think you are missing the point, it is not a case of just the glue failing.
Is is bad engineering practice to cut/reduce/or otherwise tamper with the load bearing item.

You should not have to 'rely on' the glue.
The load should be carried by the spar with other items 'glued to it'
Unfortunately at the point of maximum load and leverage the spar is weakened by cutting it to fit to the rib.

The glue is on a but joint which is not good.
If you HAVE to join a spar ( or any load bearing structure) you should use an angled slice on both parts to increase the glue area considerable. Usually this is around 30 degrees, from the edge.

I ask you, if you were building the plane and the balsa spare was not long enough, would you Butt joint another piece to it?
I am sure the answer is no.
Now, if i ask you to do that at the fulcrum point where the tail-plane is applying maximum force as it moves up and down and twists! I know the answer would be no.