NextCloudPi: Cannot restore database

I have had a problem with my Nextcloud instance for some time. It was running with NextCloudPi on a Raspberry Pi 4 and one day my entire network stopped working. All network devices connected to the same LAN port of the router were no longer accessible. It turned out that the reason was the Pi, but I still don’t know why. In any case, it couldn’t get past the rainbow screen with the display connected.
Not so bad, I thought to myself, installed NextCloudPi on another SD card and wanted to use ncp-restore to restore the backup that was on the external hard drive. The backup only contains the NCP configuration and the database, but no files! Previously, when NextCloud updates failed, the import was a matter of minutes or several hours, this time it took several days and finally failed with the following message:

restore database...
ERROR 2013 (HY000) at line 3449: Lost connection to MySQL server during query
Error restoring nextcloud database

A look at the database shows that only the tables up to oc_filecache are present. There are about 1.3 million rows in this last table. In a first look I was able to find files there that are in the .opcache folder. Should something like opcache files should really be in the database?

Does anyone have any idea how I can get my instance up and running again?

That would help me a lot!

For clarification: I still have the old sd card with the database on it, but I cannot boot from it