Hello Till M,
I'm sorry to hear that you're having trouble activating your Windows Server 2019 Standard license. Error code 0xC004C003 usually means that the MAK key you're trying to use has been blocked by Microsoft. That is, your MAK Key has reached the limit of the number of times. To resolve this issue, you will need to get a new MAK. You can reach Microsoft support by phone or chat through the Microsoft Support website. For more information about phone activation, please see Volume License Key Phone Numbers | Microsoft Volume Licensing.
Or you should type the retail key correctly which you have purchased to activate.
Notice: OEM key refers to the Windows product key pre-installed on a new machine, it is tied to the hardware and cannot be transferred to another machine.
Hope it is helpful for you.
Kind regards,
Lei