Thanks Mafoster. This worked for me as well, at least for the last 24 hours.
The second time my server locked up, I could see the screen of my linux virtual machine and I could move the mouse around but clicking didn't do anything. I was able to see my shares on my macbook, and I was even able to telnet into the server and try a reboot, shutdown and running diagnostics through the command line. Nothing worked. In the end, I had to force shutdown with the powerbutton. I have attached the diagnostics after starting the server again, just in case someone can put them to any use for troubleshooting.
Cheers
Z
diagnostics-20170611-2018.zip