Because there is a directory in my albumtree that is not deletable I want to start all over.
I want to have the situation like I just installed ZenPhoto.
Becauseeverything is new to me I didnot make a Backup yet.

What steps should I perform to return to the 'factory defaults'?

Thanks

You should know that Zenphoto is file system based so it would be important to know why you cannot delete an album. On the file system it could be an permissions issue.

The zenphoto and server error logs should tell you something.

Factory defaults would mean to clear the database and remove all albums on the filesystems, then re-run setup. If you changed any files like theme ones you need to re-upload the files as downloaded.