I suggest reinstalling package called "plymouth-theme-ubuntu-logo". it's the default plymouth theme used in ubuntu.
sudo apt-get install --reinstall plymouth-theme-ubuntu-logo
Then make sure you are choosing it.
sudo update-alternatives --config default.plymouth
Then you will choose one of the installed themes (choose ubuntu-logo):
There are 4 choices for the alternative default.plymouth (providing /lib/plymouth/themes/default.plymouth).
Selection Path Priority Status
------------------------------------------------------------
0 /lib/plymouth/themes/numix/numix.plymouth 100 auto mode
1 /lib/plymouth/themes/numix/numix.plymouth 100 manual mode
* 2 /lib/plymouth/themes/solar/solar.plymouth 10 manual mode
3 /lib/plymouth/themes/ubuntu-logo/ubuntu-logo-scale-2.plymouth 99 manual mode
4 /lib/plymouth/themes/ubuntu-logo/ubuntu-logo.plymouth 100 manual mode
Press enter to keep the current choice[*], or type selection number:
Then save changes
sudo update-initramfs -u
Reboot, and check if it was solved