Your offer is very kind. Thanks for this. To my regret I am not technical advanced enough to determine what went wrong. I installed the plugin two times on my wp multi site network to make sure it is not an event by chance.
The error after pressing the done button in settings screen is: The requested URL /wp/wp-admin/network/admin-post.php was not found on this server. Indeed the file does not exists but e use a standard wordpress version (4.2.2) so maybe the plug in is not compatible?
Meanwhile i have been able to restore the data base by using a former backup from a different tool. If it helps with your development the error message in phpmyadmin was along the line drop database because ahm file was discovered