What is this?

I already disabled the plugin by renaming the folder. So I can log in. But now the website is full of errors https://www.hairmakeupphuket.com/

Hi, that error message is not from the Cornerstone plugin hosted here in WordPress’ official plugin directory.

It sounds like you may be using a plugin from a different source that used the same or similar name. The plugin may have included support information on the plugin’s own admin page. You may also be able to find information on getting support via the site where you downloaded/purchased the plugin.