Admin Tools" plugin is enabled. I clicked on Extensions / Plugin Manager / System - Admin Tools and set Ordering: 0 Order First. I tried to save, but the System - Admin Tools didn´t closed, the abort-button didn´t worked too.
Finally I had to log out, I logged in again. I tried to open the System - Admin Tools Plugin again, but it was not possible anymore. I couldn´t open it again.
Only once a hint appeared at top of plugin list, which said, it is not possible to get direct access to this plugin (sorry, I dont know the correct words anymore). The hint didn´t appeared again.
By the way: When I sort plugins by ordering - it is not first of the System plugins (sort by ordering.).