@luckow I'll give it a whirl and let you know....
I've uploaded the zip (Drupal-9.1.4.zip).
Then I just went to the url.
Had an immediate error:
The directory sites/default/files does not exist, so I made it, as well as the copying the missing settings.php.
I then had to change ownership of the files directory, recursively.
Then the changes were recognized by the web installer and I went to the next step.
I entered the MySQL info from the LAMP-provided info, and the next install step started.
I was then brought to the Configure Site page, entered all the details, pressed ok... and
The same message you received!
So, I thought I could trick myself into the site and so I appended an admin url to my domain, /admin/config/system/site-information, and what do you know, my site's admin page loaded.
Interestingly, none of my details which I entered in the Configure Site page were displayed, and the home page was actually https://mydomain/node, so I removed the /node.
I then tried to visit the root domain but got a new url, https://mydomain.com/user/1, so something is still out of wack.
I then shortened the admin url to just /admin, and that page loaded. In fact, I could change the theme, and do a bunch of stuff, so Drupal is installed. It might be a permissions thing, so I am going to post this for now while I look around more.
OK, my site is now working. Unfortunately I did a few steps, any of which may have done the trick.
Going in reverse:
Someone at https://www.drupal.org/forum/support/post-installation/2016-12-06/the-website-encountered-an-unexpected-error-please-try suggested appending /update.php to the root domain as a means to trigger some kind of refresh or update. I did it, and it seemed it was basically a database update. Once it was finished, my site was accessible.
However, before that I hand-tweaked the settings.php file, and activated a few settings:
#$settings['config_sync_directory'] = '/some/directory/you-chose was like this, so I removed the # and added the actual config sync directory. Seemed important.
I removed the # from
$settings['allow_authorize_operations'] = TRUE;
I removed the # from
$settings['file_public_path'] = 'sites/default/files';
Any or all of these steps might have done the trick, but I only restarted the app and checked if it was loading after adding /update.php to the domain, https://mydomain.com/.
For what it's worth, I tried restarting the app several times from the Dashboard, but strangely enough I couldn't tell if it had or not. After I had edited those 3 above, and before I added /update.php I had restarted the app from within apps File Manager window - there are three horizontal dots beside "UPLOAD" which give you three options, one of which is Restart App. I restarted the app from there and there was an immediate header at the top of the browser window telling me the app was restarting. Maybe it hadn't before.