How to survival guide Content Server 7.5 / KPro

Dear Experts,

I am a bloody beginner with the SAP Content Server software.
The installation with SWPM was a breeze:

1) Install the Content Server software
2) Install the MaxDB database

And now the pain started:

1) Invoking the test URL:

https://fqdn:1091/sapcs?serverInfo

renders 500 Internal Server Error

2) https://itsiti.com/steps-to-install-configure-sap-content-server-7-53-file-system-on-linux-suse/

Here they advise to start the configuration by launching the following URL:

https://fqdn:1091/sap/admin/public/default.html

However, this first opens a login dialog and I've no clue and can't find any hint on how to login here :-(
The OS user <SID>adm is not accepted :-(

3) I have seen that in the work directory in the logs, there are some TLS and certificate errors. Probably this
is why HTTPS is not working. As I did not choose to activate HTTP during the installation, I decided to do it
manually, by setting manually in the profile:

icm/server_port_1 = prot=http, port=1090

and restarted the CS instance, however when opening

http://fqdn:1090/sapcs?serverInfo

it still renders 500 Internal Server Error :-(

Please kindly advise on how to tackle this. Any URL to any Administration Guide would be warmly welcome too, as
I was unable to encounter any official administration documentation URL / PDF :-(

Many thanks in advance!