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I am new to Sendy, (thinking of moving away from PHPList), and have a fresh CentOS8 + LAMP + Wordpress VPS.
After setting up the MariaDB database and user, and filling the info into Sendy's config.php, I visited mydomain.com/sendy for the first time to see the installation page is without any styling, (I don't have a screenshot of it, I only have one of the login screen which came later).
At the installation page, I could see that one prerequisite was not fulfilled, it was about mod_rewrite.
I checked LoadModule rewrite_module modules/mod_rewrite.so was present in /etc/httpd/conf.modules.d/00-base.conf
LoadModule rewrite_module modules/mod_rewrite.so
And I set AllowOverride All for <Directory "/var/www/html"> in /etc/httpd/conf/httpd.conf
And I gave sudo systemctl restart httpd
sudo systemctl restart httpd
When I reloaded the installation page, there was no improvement, so I filled all fields except the 2 amazon fields, because I had not set that up yet, and I pressed INSTALL.
Then it brought me to the un-styled Login page.
I visited your Sendy installation and see that the login page is loading fine. So I guess you've figured out the APP_PATH was probably set incorrectly.
Yes, I changed the http:// to https:// in config.php .
Figured it out after using browser developer tools to see that the errors were about trying to request items via http. Mixed Content errors.