After a few days of experimenting, it seems the problem comes from the mouse or capturing the mouse in a VM without guest additions.
In the old version of VB, before installing guest additions, the mouse would behave strange within the guest window.
When capturing the mouse in the guest, it would slow down to about 1/2 speed, it would show up in the very bottom left corner of the guest and when moving over the window after about 1/2 the distance to the other side the mouse pointer would freeze within the guest and a 2nd pointer would appear outside the guest, meaning the top half and the right half of the guest window were not reachable for the mouse without guest additions.
In the new version of VB this problem didn´t exist on start, whether captured in the guest window or outside,the mouse would behave the exact same, but after installing guest additions the manager would crash upon closing the VM.
After Windows has applied "compatibility settings" to the manager, the mouse behavior is the same as in the old version of VB (a mess without guest additions and working fine with guest additions installed) and the manager doesn´t crash anymore.
Anyone care to explain?