So I deinstalled Akeeba Backup and reinstalled again. Afterwards I installed Admin Tools twice. After I enabled the plugin for Akeeba Backup Update Check Admin Tools showed a blank page again.
.htaccess was disabled and enabled, the config.php was changed as I read in your advise for someone who had the same problem - nothing happened. The Problem appears to be Akeeba Backup.
Do you have any solution?
Thanks, Werner