@Hypolite Petovan

After a break I started to check out everything again and did several tests.
I changed the order of the lines in local.config.php and managed to get the exact same lines and order in the bin/console prints of both sites.
The problem stayed.
Than I checked example_local.config.php to compare and found that actually any of the lines figure there, so I deleted them all together.
Right now local.config.php looks like this:
'config' => [
'admin_email' => '[email protected]',
'sitename' => 'sitename',
'register_policy' => \Friendica\Module\Register::CLOSED,
'register_text' => '',
],

'system' => [
'ssl_policy' => 1,
'default_timezone' => 'UTC',
'language' => 'es',
],

The site still behaves the same way. At first it loads correctly, and quite fast, after some minutes it starts taking lot's of time and ends up like in the screen of the initial post.

Now the bin/console config print shows only the following two specific lines related to the whole path issue, all the other references mentioned above are gone.
I have no idea how to delete the remaining references. Those lines have to figure somewhere in the DB I guess, but again, no idea what lines to execute to display them or delete them:
config.system.basepath => /
system.basepath => /var/www/subdomain