You are missing the "--auto-attach" option from your registration command...
[root@rhel8 ~]# subscription-manager register --username=dominik.holler --password=xxx

Append "--auto-attach" to the end of that ^^^^ command and you will be successfully registered and subscribed in one step.


The following command that you ran....
[root@rhel8 ~]# subscription-manager auto-attach
Auto-attach preference: enabled
... did not perform an action, it simply returned a preference.  Read the man page for more info on the auto-attach module.  With this preference, your system will eventually execute "subscription-manager attach --auto" within the next four hours and your system will automatically be subscribed. 


Alternatively, had you run the following command after registration, you would have been immediately subscribed...
[root@rhel8 ~]# subscription-manager attach --auto



Yes I know... "attach --auto" and "auto-attach" sound the same but they are not.

(In reply to John Sefler from comment #1)
> You are missing the "--auto-attach" option from your registration command...
> [root@rhel8 ~]# subscription-manager register
> --username=dominik.holler --password=xxx
> 
> Append "--auto-attach" to the end of that ^^^^ command and you will be
> successfully registered and subscribed in one step.
> 
> 
> The following command that you ran....
> [root@rhel8 ~]# subscription-manager auto-attach
> Auto-attach preference: enabled
> ... did not perform an action, it simply returned a preference.  Read the
> man page for more info on the auto-attach module.  With this preference,
> your system will eventually execute "subscription-manager attach --auto"
> within the next four hours and your system will automatically be subscribed. 
> 
> 
> Alternatively, had you run the following command after registration, you
> would have been immediately subscribed...
> [root@rhel8 ~]# subscription-manager attach --auto
> 
> 
> 
> Yes I know... "attach --auto" and "auto-attach" sound the same but they are
> not.

subscription-manager attach --auto did the trick.
A stupid error of me, thanks.

But is this by intention, that the VM is now subscribed to a subscription for physical machines?
[root@rhel8 ~]# subscription-manager list --consumed | grep "System Type"
System Type:         Physical
[root@rhel8 ~]# virt-what
kvm

(In reply to Dominik Holler from comment #2)
> But is this by intention, that the VM is now subscribed to a subscription
> for physical machines?
> [root@rhel8 ~]# subscription-manager list --consumed | grep "System Type"
> System Type:         Physical
> [root@rhel8 ~]# virt-what
> kvm

YES - A subscription with SystemType:"Physical" is always consumable by a virtual system when necessary.  In this case, the only subscription available in your account that provided content for the installed engineering product "Red Hat Enterprise Linux for x86_64 Beta" was SKU:RH00069.  Therefore the auto-attach algorithm in the entitlement server (Candlepin) has no other choice but to attach an entitlement from the physical pool.  If there was two available pools (one of SystemType:"Physical" and one of SystemType:"Virtual") that both provided the installed engineering product "Red Hat Enterprise Linux for x86_64 Beta", then the auto-attach algorithm in the entitlement server would have attached an entitlement from the virtual pool for your kvm guest.

John, thank you very much for your hints!