"Not Found The requested URL /~user/admin was not found on this server"

My problem is just like the one described in /forum/archive/the-requested-url-admin-was-not-found-on-this-server-t9024 but that thread prematurely ends with a solution.

The site works initially (both grav-admin and grav-skeleton-onepage-site) until I install the admin plugin either manually or via gpm. Thereafter I get "Not Found The requested URL /~user/admin was not found on this server." until I remove the plugin.

I have read the pages https://learn.getgrav.org/15/troubleshooting/page-not-found , https://learn.getgrav.org/15/troubleshooting/htaccess and https://learn.getgrav.org/15/troubleshooting/permissions .

I have:

1. Set permissions using the script example provided on theses pages
2. Ensured that .htaccess existed and tested it with the .htaccess_tester.php script provided on these pages
3. Checked /etc/apache2/mods-enabled/userdir.conf for AllowOverride All

It really seems to be a rewrite issue but my knowledge falls short of a solution.

My server: Debian 8 Jessie; Apache2; PHP 5.6.40-0+deb8u2

I've been working at this since yesterday and I'm almost bald from pulling my hair out! I've been so psyched to move my site from Wordpress to Grav after days of CMS research but I'm running low on steam. 😐

Can anyone help with further advice before I retreat in dismay and disgrace back to the world of Wordpress? 🙄

After carefully going through all my Apache2 config files, ensuring my files were in the configured userdir and not in a grav* sub-directory and ensuring my files' permissions were set according to the Grav documentation recommendations, my latest results are:

On bowser: "The requested URL /home/user/public_html/index.php was not found on this server."
Apache2 log: "[rewrite:trace1] [pid 4475] mod_rewrite.c(475): .... [perdir /home/user/public_html/] internal redirect with /home/user/public_html/index.php [INTERNAL REDIRECT]"

I know the file /home/len/public_html/index.php exists and the server definitely has access to it as when admin plugin is removed the site renders in the browser. What does this mean ... anyone?

🥺