Error message "Failed to start Load AppArmor profiles" at boot time

I have tried to upgrade Ubuntu from 18.04 bionic to 20.04 focal with the update manager. The process hanged for ever when installing a package called "libgdk-pixbuf" or something similar. So I had to restart the computer. After starting the upgrade process again, this time with the command line "$ apt upgrade", I could not boot. There was an error message:

"Failed to start Load AppArmor profiles."

Could someone please help me to repair the system?