[Plugin: NextGEN Gallery] Lightbox-Effect doesn´t work

Dear Community!

I´m new to blogging since I installed WordPress 2 months ago. I think it´s not that difficult because everything is explained in detail.
Unfortunately I have still problems with NextGen-Gallery although it´s a great plugin when it works.

When I insert a gallery in a post, it shows only a list with the thumbnails. On click he doesn´t open the Lightbox. Instead the picture is shown in the browser (Firefox 3.0.7). Look at my blog: http://www.cf-net.de

I would be great if there´s is someone who can help me!!

Here are my settings:

Server Settings
* Operating System : Linux
* Server : Apache/2.2.3 (Debian) mod_python/3.2.10 Python/2.4.4 mod_ssl/2.2.3 OpenSSL/0.9.8c
* Memory usage : 32.75 MByte
* MYSQL Version : 5.0.32-Debian_7etch6-log
* SQL Mode : Not set
* PHP Version : 5.2.0-8+etch13
* PHP Safe Mode : On
* PHP Allow URL fopen : On
* PHP Memory Limit : 32M
* PHP Max Upload Size : 2M
* PHP Max Post Size : 8M
* PHP Max Script Execute Time : 30s
* PHP Exif support : Yes ( V1.4 )
* PHP IPTC support : Yes
* PHP XML support : Yes

Graphic Library

* GD Version : 2.0 or higher
* FreeType Support : Yes
* FreeType Linkage : with freetype
* T1Lib Support : Yes
* GIF Read Support : Yes
* GIF Create Support : Yes
* JPG Support : Yes
* PNG Support : Yes
* WBMP Support : Yes
* XPM Support : No
* XBM Support : No
* JIS-mapped Japanese Font Support : No

I have installed the newest version of NextGEN (1.2.1) but I tried older versions (0.96) as well. I use WordPress 2.7.

I am thankful for any hint!

Best wishes,
Cife24