Search does not work – url problems

hello to all
I have set up wp to use as an CMS on this page: Website WSNH

The customer wanted the changing pictures before the blog, so I use index.html with http-equiv=”refresh” to switch the pics.

Now they told me, that the search is out of order. It always results in index.html.

I think this is an url problem.
http://wasserschloss-neuenheerse.com/?s=Afrika&search=suchen does not work, but wasserschloss-neuenheerse.com/index.php/?s=Afrika&search=suchen do.

So I try to change action="<?php bloginfo('url'); ?>/" in searchform.php, but I’m not sure how I have to do this. I tried
action="<?php bloginfo('url'+ '/index.php'); ?>/" but it does not work. Any suggestions?

http://wasserschloss-neuenheerse.com/index.php?s=Afrika&search=suchen