I have added a new Amazon EC2 cloud at the "Configure clouds" page. After adding my EC2 credentials, the "Region" drop-down keeps empty. After some investigation, I added "https://ec2.eu-central-1.amazonaws.com" as Alternate EC2 Endpoint. After that, the Region dropdown is filled and i can select my region (eu-central-1). Now i save my changes and all works as expected.
When I now open the "Configure clouds" page, The Region and also the Alternate EC2 Endpoint are both empty. When I now change somethind (i.e. InstanceType of an AMI) and click "Save", i can not longer run any instances, because it seems that an empty region was saved (yes, dropdown was empty)