Yes, the framework was folder in the root directory. You know what I did? I found this on Google SapphireTest.php at Github.com under https://github.com/silverstripe-australia/silversocial/blob/master/framework/dev/SapphireTest.php and have them copied to the old file. Now everything works and the website and the blog you can call. I am so glad that you gave me suggested this idea with SapphireTest.php.
Now I want to remove from the blog comments the NoFollow attribute. I'll probably open a new thread here in the forum.
You tell me, you have to install the plugin after each command dev / build run? I hope that I will have no more problems. Thank you for your time and help.