After installing Windows Server 2019 Standard, I tried to activate it with a MAK key from the Microsoft Volume Licensing Portal. I clicked on "Change product key" and entered the key from the Microsoft Portal. I got the following error message:
The product key you entered didn't work. Check the product key and try again, or enter a different one. (0x80070490)
Searching on Google, this error number mostly refers to Windows Update or Windows Store errors, but not to licensing issues.
I finally tried to activate via the command line, by using these commands:
slmgr.vbs -ipk XXXXX-XXXXX-XXXXX-XXXXX-XXXXX (slmgr.vbs -ato)
This worked right away.
Update: Several readers commented that only the first command is needed. I put the second one in brackets for that reason.
So this is just a GUI activation problem on Windows 2019.