Hej, thanks for the fast and appropriate help.
It really (I can´t believe it) was the mod_rewrite not enabled.
I really thought the other things (other gallery and the lightweight php framework codeigniter) uses also mod_rewrite, but they don´t.
It was not loaded. I take a look at the phpinfo() and it was not listed.
Thanks for your link.
A note to the troubleshooting page: it would be nice if someone can add some sentences to apache like this:
"When the apache module mod_rewrite is not loaded, you can´t use mod_rewrite within your ZenPhoto configuration. Via phpinfo() you can look, if mod_rewrite is listed as loaded module. If is not, you can load it by open the httpd.conf of your apache and uncomment (delete the leading #) or add this line:
LoadModule rewrite_module modules/mod_rewrite.so"
Thanks!