Hi Dominique, sorry I missed this post last night.
As far as I am aware, yes you should be able to rename the page and as Sandra mentions, you will then just need to make sure the links are to the correct names.
I am guessing that the numbers in this case were added by the sitebuilder, it does this sometimes (especially with free sitebuilders) when you have viewed the page before saving it.
Also though I notice your index page -
http://www.blueheart.dreipage.de/index.html - doesn't seem to be your very first page, this might cause problems too. You should always try to have your index.html as the very first page of your site.
Are you using frames? Looking at your source code I can see an iframe in there. Again, this might make things more complicated.
Please, if you are still stuck, give me a shout. I may be able to help if you can show me the sitebuilder you are using.