500 error w/installer.php even w/manual extract

Trying to copy/move WordPress from mydomain.com to mynewdomain.com

1. Since mynewdomain.com is not currently hosted at the new host, I edited my local hosts file:
12.345.67.890 mynewdomain.com
2. Tested using an index.html at mynewdomain.com. It worked. Moved it out because the target folder is supposed to be empty when using Duplicator
3. Using Duplicator, created the zip and installer files in mydomain.com/wp-snapshots
4. Copied the two files to mynewdomain.com using cPanel File Manager, renaming the installer to just installer.php
5. Went to mynewdomain.com/installer.php in the browser

500 error in browser
Chrome dev panel just says:
Failed to load resource: the server responded with a status of 500 (Internal Server Error)

Nothing relevant in the Errors file in cPanel

Just recently installed an .htaccess file in public_html, which mynewdomain.com and mydomain.com are in subfolders of. Would this cause a problem?

RewriteEngine on
RewriteBase /
RewriteRule ^$ mynewdomain.com/ [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ admin/$1 [L]

Tried manually extracting the files and running again. Still 500 internal server error.

Tried in FireFox and got: “The character encoding of the HTML document was not declared. The document will render with garbled text in some browser configurations if the document contains characters from outside the US-ASCII range. The character encoding of the page must be declared in the document or in the transfer protocol.”