Hello Michael,
I haven't tried to contact my local customer support during festive period, but I did solve the problem. It turns out that I have been using the wrong credentials from the beginning. The hardest part for me was to realize that my MS account which I used for upgrading the Plan and all other MS services doesn't have rights to create Subscriptions in Azure. I was confused because I thought my MS credentials correlate with the one user I had in my Azure AD Tenant which had the same name, same email address, same description but the key is different username (principal name). That user had the Global Administrator role from the beginning but I've added Billing Administrator role also. When I logged in as user@Stuff .onmicrosoft.com, the subscription creation was done in 10 seconds without any errors.
I am still bothered how these two correlate and it would be helpful if you could link some literature to further my understanding.
Thank you in advance and best regards,
Marko