Spontaneous failure of manager, followed by error 500 on upgrade

Yes, replace the /core/ with your site's path from that file. Clear the cache, and attempt setup again.

But first, let's put your old files back as existing, and re-unpack MODX on the server making sure that you're not overwriting. Maybe I've been lucky with the method I use to upgrade, and your server actually did overwrite the files. Sorry about the confusion. But since you've got a copy of the old stuff, you can begin anew.

There's a few files that you'll want to check and make sure the paths are correct after you "merge" the new files. Essentially, you need to follow the directions at Moving your site to a new server, but here's an excerpt:
http://rtfm.modx.com/display/revolution20/Moving+Your+Site+to+a+New+Server

Updating your Config Files

Once you've deployed files to the new server, you need to update the main configuration file: core/config/config.inc.php. You have to update paths to 6 different resources. Open the file and update the values for the following variables doing a find and replace:

/* PATHS */

$modx_core_path= '/path/to/modx_doc_root/core/';
$modx_processors_path= '/path/to/modx_doc_root/core/model/modx/processors/';
$modx_connectors_path= '/path/to/modx_doc_root/connectors/';
$modx_manager_path= '/path/to/modx_doc_root/manager/';
$modx_base_path= '/path/to/modx_doc_root/';
$modx_assets_path= '/path/to/modx_doc_root/assets/';
 
/* HOST (used for command-line PHP stuff) */
$http_host='yoursite.com';

If you are also moving your site into or out of a subfolder, be sure to update the variables such as $modx_connectors_url, $modx_manager_url, and $modx_base_url. They should generally
end with a slash (e.g., $modx_base_url='/' for a site not in a subfolder).
Permissions
Before you can edit your config file, you may need to loosen up the permissions. After you've edited it, be sure you restore the read-only permissions on the file.

There are also 3 additional configuration files that contain two PHP constants:

 define('MODX_CORE_PATH', '/path/to/modx_doc_root/core/');
 define('MODX_CONFIG_KEY', 'config');

/config.core.php
/connectors/config.core.php
/manager/config.core.php

Make sure you update the paths in these files as well.

"But first, let's put your old files back as existing, and re-unpack MODX on the server making sure that you're not overwriting. Maybe I've been lucky with the method I use to upgrade, and your server actually did overwrite the files. Sorry about the confusion. But since you've got a copy of the old stuff, you can begin anew."

Wait -- what?

I'm not sure what you're asking. The old folders are a previous revision (not too far, about three or four months, I think, but definitley a different number).

The current files, which I've loaded into the site twice now, are the most recent version.

The old version was broke.

The best I could do to get it back was only the home page, but I recall being told that it's not only folder names and contents, but changes to the database.

So, you're asking me to switch back to old files with the updated database?

I... I'm confused.

If the new version can read the db backup (which I made via the PHPMyAdmin tools, to the best of my knowledge), then why not do (at this stage) a complete fresh install, and then re-import the db?

I admit this is unsure footing to me, but based on my lack of concrete knowledge, it seems like that's what we're tying to do in a roundabout fashion.

It's hard to know what you've got there, sorry.

If you install fresh and import the db it should all show up as it was before. I was concerned that your files got overwritten but it won't matter if you do a fresh install.