I had the same issue after PHP update, not upgrade. Was able to solve it with help from spatie git-hub page with below instruction.
It seems this error started happening for a lot of people last week. No real reason why though. To partially fix...
Step 1:
sudo nano /etc/ImageMagick-6/policy.xml
Step2:
Add/edit the following lines
<policy domain="coder" rights="read|write" pattern="PDF" />
<policy domain="coder" rights="read|write" pattern="LABEL" />
You may also need to comment out the MGV line if you have it. I did not.
Step3:
Restart web server AND php-fpm. i.e. (on nginx/php7.2)
sudo service nginx restart
sudo systemctl restart php7.2-fpm
Follow the below link for more details if the above 3 steps does not work for you.
https://github.com/spatie/laravel-medialibrary/issues/1268#issuecomment-429928562