Why does a VirtualBox error completely locks my ubuntu desktop?

Question

I'm using VirtualBox (Version 5.1.38_Ubuntu r122592) under lubuntu (Linux hostname 4.4.0-148-generic #174-Ubuntu SMP Tue May 7 12:20:14 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux) to start turbo debug under a DOS 6.22 VM.

When I start td.exe VirtualBox hangs and completely locks my desktop.

Here the screenshot:

The log indicates:

00:00:24.256603 WRSETUP  EXE        12,800 04-28-18   3:14a
00:00:24.256603        85 file(s)      8,004,291 bytes
00:00:24.256603                       61,132,800 bytes free
00:00:24.256603
00:00:24.256603 C:\TASM\BIN>td
00:00:24.256603
00:00:24.256603 --------------------------------------------------------------------------------
00:00:24.256603   !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

System information:

NAME="Ubuntu"
VERSION="16.04.6 LTS (Xenial Xerus)"
ID=ubuntu
PRETTY_NAME="Ubuntu 16.04.6 LTS"
VERSION_ID="16.04"

Virtualbox installation information:

virtualbox:
  Installed: 5.1.38-dfsg-0ubuntu1.16.04.3
  Candidate: 5.1.38-dfsg-0ubuntu1.16.04.3
  Version table:
 *** 5.1.38-dfsg-0ubuntu1.16.04.3 500
        500 http://us.archive.ubuntu.com/ubuntu xenial-updates/multiverse amd64 Packages
        100 /var/lib/dpkg/status
     5.1.38-dfsg-0ubuntu1.16.04.2 500
        500 http://us.archive.ubuntu.com/ubuntu xenial-security/multiverse amd64 Packages
     5.0.18-dfsg-2build1 500
        500 http://us.archive.ubuntu.com/ubuntu xenial/multiverse amd64 Packages

Do you know how to solve this issue? What's the reason?

Thanks in advance.