Hej Hannes,
Spot on.
On the first trial I received a warning that AWS had to verify my account. This took some time and I assume that the large instance was only enabled after the verification was done by them.
I noticed that my account has 1 instance r3.8xlarge enabled. So contacting AWS support was not needed.
I created a new instance of the solution - on the AWS console the large instance is now being created.
Thanks for the hint !
// Frank