I now have a vague idea, what may have happened.

The really strange thing is that I did not write to this venti at all (at least not intentionally).

Now I tried to repair the config by deleting  the last arena parttion, reformat a new one and buildindex from scratch.
After some time, when I used venti/copy to copy a tree to another venti, the last partition filled up again.

I noticed that the venti.conf stored in the first arena partition was not actualised.  Is this the cause of the strange behaviour?
The http storage and index listings showed nothing which could alert me.