VM crashes (aborted) right after entering unity login password

[chankim]

I have installed virtualbox(Version 6.1.8 r137981 (Qt5.6.1)) on my ubuntu machine(16.04.6 LTS, kernel 4.15.0-101-generic).
and I made a VM with the same ubuntu 16.04.6. It was successful and I logged in to it and tried building ubuntu following BuildYourOwnKernel web page(you can google it, I can't put it here since I became a member today.)
I managed to build ubuntu and kernel, and then I thought I'd better have the shared folder with the host. After tyring it, (I did the 'Insert Guest Additions CD Image ..' of the 'Device' menu, set it to auto mount, having specified the host folder location and guest mount point), I saw the shared folder is not working. and then I remember I turned the VM off and turned it on. Then, when I entered the password in the unity login window, the VM just crashed with 'abored' state. Tried it 3, 4 more times, the same. Where should I look into?
(I upload the log file)

[chankim]

I removed the 6.1.8 version and installed 6.0.22(?) version. Now the problem is gone, the old VMs run ok.
looks like version 6.1 has a problem.