High Sierra: qmasterd/opendirectoryd: Too many corpses being created

Hi all, on our family white Macbook (2009), which used to work fine, we have since yesterday a spinning boot.

In verbose mode can be seen:

Waiting for DSMOS...

Process[187] crashed: qmasterd. Too many corpses being created.

Process[188] crashed: opendirectoryd. Too many corpses being created.

Disk tool first help and fsck are ok, I did not see errors in the details when I ran it.

I can also find thru the harddisk without issues at the recovery console.

So the filesystem is ok, and since it can boot in recovery mode, I assume hardware and filesystem are generally ok. Disk space is plenty.

Creating rc.server did not help. I reinstalled OS without a resulting change.

/var link also seems ok. I can change directories and ls in single-user mode, but not create files since mounted in read-only.

I removed the Mac from my WLAN (Fritzbox), so it cannot connect any more. No change.

I renamed the directores of the virus scanner as an attempt to isolate that. No change.

Attempts to start in safe mode are also unsuccessful. If anyone has a hint to start in safe mode with verbose, appreciated.

I don´t want to USB attach the disk to my other MPB to backup, and to later reformat/reinstall, without understanding the root cause of this.

I saw quite few other discussions on similar topics, without having found the right hint or solution.

Can you please help?

Many thanks / Stefan