Win 10 guest crashes virtualbox and host freezes

[otto_unnormalverbraucher]

I am running a Win10 64bit guest on an Ubuntu 18.04.1 64bit host. Ram is 16GB host / 6GB guest and vdi-disk is 100GB of which about 80GB are used. Physical disk is 500GB SSD. Virtualbox is 5.2.18.
Everything went smoothly until I updated from Ubuntu 16.04 LTS to 18.04.

Everytime I start the VM the guest boots up slooooow (about 2-3 minutes until it reaches the login screen).
Sooner or later the VM freezes (earliest, even before the Windows login screen appears, latest, when logged on and trying to update guest additions) and in 4 out of 5 cases also the host is frozen.
If the host is not crashing the guest just dissapears from screen i.e. the process dies.
When the host is frozen, I can in no way reactivate it. Keys and mouse are not producing a result.

Here is what I tried: [Host]+F, ALT+CTRL+Backspace (yes I know, not working on newer machines anymore), ALT+Tab, CTRL+ALT+DEL, tried to activate a terminal, tried to switch to another console. All to no Avail. The only thing that "works" is to restart the host with ALT+PRINT and the sequence r+e+i+s+u+b, which leads to a reboot of the host. This indicates, that the keyboard is functional, but Host and guest are completely frozen.
I tested, if other VMs behave in a similar way, but each and every other VM works well.
Also, if I run the W10-VM headless, It does not affect the host in any way, but boots up until it reaches the login screen. I can connect, but then it will freeze like it did, when run with gui. If I detach fast enough, the process will just die.
I searched the log for Errors or exceptions w/o result.
The Tips to disable Drag&Drop did not help, also I do not have tightvncserver installed as mentioned in other posts here. (Sorry not allowed to post URLs, because to new in the forum)

Can anybody direct me to a solution for this problem. Any help will be greatly appreciated.