Cannot restore backup as system can't connect to local MySQL Server

I have NCP on a raspberry pi. I created backup and sd image before updating to NC 16 and buster.
Today I had a power outage, could not ssh into the device again.
I copied the sd image to a new sd card. With that I can access ncp-admin.
This shows Nextcloud check error and MariaDB down.
I tried nc-restore to restore the last backup from the same day as the sd image, but that fails with
ERROR 2002 (HY000): Can’t connect to local MySQL server through socket ‘/var/run/mysqld/mysqld.sock’ (2)

What is wrong, how to fix that?

Did you try to restart the mysql server?