Lokal kein Zugang zum Installer, Panel und Login (de)

when I hit install the empty dialog box appears (see screenshot above).
I then hit “OK” to dismiss this dialog and get back to square one.
Whether I reload the “install” screen or explicitly navigate to “/panel” or “/panel/login” I get the login screen, but can’t login (2nd image above). It tells me “Invalid Login” despite all account files exist.

While the login screen is open, PHP’s error log shows
The file does not exist at the given path: "bla/site/templates/error.php"
and The file does not exist at the given path: "bla/site/templates/error.html.php" respectively.

I also ran Firefox with a squeeky clean profile because: why not.
With the Login screen open, I now noticed this in the console:
"The JSON response from the API could not be parsed. Please check your API connection." followed by some backtrace for “app.js” and others.
The message keeps repeating; maybe due to some XHR polling or a rejected Promise.
I didn’t see this in my regular FF profile.

I remember this message only being shown once on the install screen in my early tests, but I got rid of it adding the HTTP PATCH to Apache. I have no idea what the culprit might be.