Hi, @paul_jan 
(Usual disclaimer: please note that I'm just another Forum user here. I'm NOT a Ubuntu developer or Ubuntu MATE developer)
I usually do the following changes in the GNU GRUB (GRand Unified Boot Loader) configuration in my Ubuntu MATE installations, that may also help you. Please, edit the /etc/default/grub file with administrator privileges, by running the following command (that will open the "Pluma" text editor as superuser):
sudo pluma /etc/default/grub
In that /etc/default/grub file, change the following lines (in your case, these lines may appear in a different order from what I show here and you will probably find other lines in between, which you don't need to change):
GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT=0
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
#GRUB_DISABLE_OS_PROBER=false
... change those lines so they become the following ones:
GRUB_TIMEOUT_STYLE=menu
GRUB_TIMEOUT=10
GRUB_CMDLINE_LINUX_DEFAULT=""
GRUB_DISABLE_OS_PROBER=false
After doing that, please save and close the file, and then run the following command:
sudo update-grub
Please, copy the output of the execution of that sudo update-grub command to some text file, so you can also include that output in another reply from you in this same discussion topic.
Now, please reboot the computer and reply later, in this same discussion topic, to tell us if this solved the issue for you