@edi0th:
I can confirm, if you have done the steps 1 to 7 in your initial post, this would run.
So I agree that this is:
I guess, that your webserver can not write in the directory “\assets\css\”. I recommend to check this, possibly with the hoster.
What is your PHP version?
What your KIrby version?
Which webserver are you using?
What is his OS?
@mauricehh:
I use this solution since Jun 3 to implement my strategy HowTo: Start Kirby development on a PC.