Blank Extras Installer page

After upgrading to MODX 2.7 I'm unable to access the packager installer (via Extras -> Installer). Error Log shows (repeated lines removed):

[2019-01-17 03:11:00] (ERROR in modProcessor::run @ /home/mywebsite/public_html/core/model/modx/modprocessor.class.php : 177) Flat file processor support is deprecated since version 2.7.0.
[2019-01-17 03:11:34] (ERROR in modTemplateVar::renderOutput @ /home/mywebsite/public_html/core/model/modx/modtemplatevar.class.php : 288) Old modTemplateVar getRender outputmethod is deprecated since version 2.2.0.

Using PHP 7.2 (I also tried 7.1)

I don't think the errors are related to the problem.

The first thing to try is manually deleting all the files in the core/cache directory and clearing your browser cache and cookies (or visiting in private or incognito mode).

If that doesn't help, it's most likely a plugin that doesn't play nice with MODX 2.7. You can disable all plugins in the modx_site_plugins table in the DB. Be sure to delete the cache files again before testing.

No, I already cleared cache, etc. I disabled (set disabled=1) all records in modx_site_plugins table. Cleared cache files again, no luck. Even disabled all package extensions, cleared cache, no luck. I had restored from a backup before upgrading. I noticed that not all package folders had corresponding .zip files, so I uploaded the missing ones. Still no luck.

The frustrating thing is there's no feedback that tells me what's wrong. MODX error log only has the "deprecated" message. And the dev tools only says 500 (no other messages).

Any other ideas?

There may be better information in the server error logs. It will help if you know the time the 500 error was thrown.