"blinking the screen" - it is X11 crashed in cycle, due to incorrect xorg config file.
Xorg config file is created automatically by Parallels Tools installer and located here: /usr/share/X11/xorg.conf.d/40-prltools.conf
It is started like this:
# Parallels Video section
Section "Device"
Identifier "Parallels Video"
Driver "prlvideo"
EndSection
................... And so on ..........
This is part of your Xorg-log file:
[ 5.973] (==) Log file: "/var/log/Xorg.0.log", Time: Fri Nov 15 20:00:48 2019
[ 5.974] (==) Using system config directory "/usr/share/X11/xorg.conf.d"
[ 5.975] (==) No Layout section. Using the first Screen section.
[ 5.975] (==) No screen section available. Using defaults.
[ 5.975] (**) |-->Screen "Default Screen Section" (0)
[ 5.975] (**) | |-->Monitor "<default monitor>"
[ 5.975] (==) No monitor specified for screen "Default Screen Section".
..................
But it must looks like this if correct xorg config used:
[ 171.280] (==) Log file: "/var/log/Xorg.0.log", Time: Tue Dec 17 15:10:55 2019
[ 171.386] (==) Using system config directory "/usr/share/X11/xorg.conf.d"
[ 171.459] (==) ServerLayout "DefaultLayout"
[ 171.460] (**) |-->Screen "Parallels Screen" (0)
[ 171.460] (**) | |-->Monitor "Parallels Monitor"
[ 171.460] (**) | |-->Device "Parallels Video"
......................
Probably, there is some application (automatic configurator) in your Ubuntu VM that remove this file (or created another config file with higher priority)