Nginx cannot be restarted due to a missing certificate

In November 2024 I stopped an app. Since then, I have never touched it again. Today I wanted to install an app, but the process stopped with Nginx Error - Error reloading nginx: reverseproxy exited with code 1 signal null.

Expected behavior: A stopped app has no effect on the restart of Nginx.

That helped in my case:
forum.cloudron.io/topic/7219/cannot-load-certificate-key--sub-domain-org-key-failed-no-such-file-or-directory?=1743936527594

And here is the “keyboard magic” in case someone has no idea how to use grep grep -rin sub.example.org .