[lubuntu] VirtualBox freezes host gui

Hi everyone.

I am running Lubuntu 18.04 and VirtualBox-5.2

When I use my Windows guest (full screen) I sometimes find that my mouse turns into a selection mode and I cannot click anywhere in the host GUI (or guest for that matter).

The only thing that I found to be working is switching consoles using Ctr+Alt+F1 and then switching back using Ctrl+Alt+F7

Any ideas what is causing that and how to fix it?

Thank you so much!