ERR_TOO_MANY_REDIRECTS with HTTPS = 'On' - vol.2

[Sulpher]

Start of the discussion: getgrav/grav#3155

the front-end works, the admin panel gets endless redirects.
If I add this line:
$_SERVER['HTTPS'] = 'on'
then the admin panel is loading correctly.

PHP Built On | Linux vh306 5.4.0-58-generic getgrav/grav#64~18.04.1-Ubuntu SMP Wed Dec 9 17:11:11 UTC 2020 x86_64
PHP Version | 7.4.10
Web Server | Apache/2.4.29
WebServer to PHP Interface | apache2handler

Info from hosting support team:
The hosting uses common work of web servers Nginx+Apache. Nginx is responsible for processing static files.

[mahagr]

Found a bug here while fixing something else: https://github.com/getgrav/grav-plugin-admin/blob/develop/admin.php#L504-L510