You must click on the "Registry" button on the left and then click on the "Scan for Issues" button ("Nach Fehlern suchen" in German). CCleaner will then scan for Registry errors.
If RegSeeker finds something CCleaner doesn't find, you shouldn't erase it - CCleaner is one of the safest Registry cleaners.
One advice : ALWAYS backup the deleted Registry keys. Always.
For good orders' sake I take the liberty to say again that the cleaning was possible (= has also showed by CCleaner) with the older versions of CCleaner. Therefore I am a bit astonished that it is not possible to make it with the newest Version.
Anythere - let's wait and see. Perhaps it will possible with a next version ...
Uninstalled the Version 2.25.1025 and reinstalled the Version 2.22.968. Result: I am able to clean in the registry "HKEY_Load-Machine Software\...\,,,\,,,\App Management\ARPCache ...
It appears to me that something has changed in the newest version or a mistake is existing in the program.
Anythere. Wait and see is probabely the right way. Or do you have a better idea?