I used the twentythirteen template, but i modified it.
I was able to solve the problem:
– I disabled the plugin
– removed the page
– removed the entry in the menu manually
– created a new page
– added the page to the menu
– actived the plugin again
Now it works. I don’t know why this happened, but i guess something was changed in the database by this Yoast SEO plugin, which couldn’t be removed afterwards.
Thank you for your help.