I’ve finally gotten past all the finagally little hidden configuration tweaks you have to make to get Concrete to install only to log into the site (v9.2.2 with the Elemental demo site) and be greeted with a popup that I can’t make go away:
I don’t know where it got that “webmaster@” email address because that does not exist anywhere. And I also can’t figure out what the actual nature of the error is. What do I do?
there may be severeal reasons for such an error, you need more informations using full debugging, see concrete5, "failed to create cache path" error when trying to load admin site - Stack Overflow
The email would be the admin login email. It’s what Concrete uses by default. You can change it in settings under email.
This looks like a webserver error so that domain is likely the domain associated with your hosting account. It might be worth reaching out to your hosting provider about configuration. You might also temporarily disable pretty URLs because it looks like it might be a rewrite error due to webserver configurations.