This question came up again here: https://wordpress.org/support/topic/error-404-not-found-44?replies=2#post-6628538
I retested the HC Custom WP-Admin URL plugin on a test site and it does now work.
Steps to add the HC Custom WP-Admin URL .htaccess code to BPS Custom Code:
The HC Custom WP-Admin URL plugin creates this htaccess code below at the bottom of your root .htaccess file when you add and save your WP-Admin slug on the WordPress Permalinks page.
Note: In this example I am using “example” for the name of the WP-Admin slug.
# BEGIN WordPress
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteRule ^example/?$ /wp-login.php [QSA,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
# END WordPress
1. Save this line of code: RewriteRule ^example/?$ /wp-login.php [QSA,L]
and delete the rest of the HC Custom WP-Admin URL plugin .htaccess code from the root htaccess file.
2. Copy this BPS root htaccess code below and paste it into this BPS Custom Code text box: CUSTOM CODE WP REWRITE LOOP START: www/non-www http/https Rewrite code here
# WP REWRITE LOOP START
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
3. Edit that code you just copied to the BPS Custom Code text box and add the line of .htaccess code that you saved in step 1 as shown below.
# WP REWRITE LOOP START
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteRule ^example/?$ /wp-login.php [QSA,L]
4. Click the Save Root Custom Code button.
5. Go to the Security Modes page, click the Create secure.htaccess File AutoMagic button and activate Root Folder BulletProof Mode.