Hi Marco,
We are aware of this bug, although it reproduces from time to time, it's hard to make it reproduce while in debug and fix it.
As a workaround restart myco, yes, even if this is not the best option this is it for now.
We will try to fix it somehow.
On the other hand in the new beta you will have the option to use middle click as selection and this problem will be out of question there.