Hi, there.
The issue was already discussed last month but since it has been closed without providing a general solution I have to reopen it as a new topic.
(Topic ID: 113922)
Outside the forum I was in contact with Christian at 20 October 2025 who confirmed the issue.
As described by Tim845, WordPress informs me that a newer version of Cornerstone is available:
Clicking either at “Details anzeigen” or “Details der Version 7.7.13 anzeigen”, which means “show details”, the links end in general at https://theme.co/cornerstone/.
Now and then I get offered the details of a differnt product called “Cornerstone” from Archetyped, as shown by Tim845.
For sure, the trial to update Cornerstone by selecting “jetzt aktualisieren” (update now) fails:
Afterwards, my website is down for a couple of minutes:
According the hints I found in searching the www, it is possible to update plugins by the same way like themes: All I need is a zip-file of the plugin, i.e. Cornerstone. Then I install it as a new plugin and the elder release is overwritten by the new one.
Wouldn´t it work out it in this way to enable me the update from Cornerstone 7.6.10? 
André