Hi Sebastian,
I have a few questions/suggestions.
1. Which version of Aras Innovator are you trying to install?
2. Was the InnovatorSolutions database created within your SQL server instance?
3. When creating a new database your input for "DB Client" should be innovator_regular. By default, all regular database requests from Aras Innovator are executed on behalf of the innovator_regular database user. Assigning this value to the innovator role will cause issues, as all innovator SQL requests would have owner permissions.
I would recommend attempting a clean install. To create a new database you'll need to erase your old InnovatorSolutions, or create a completely new DB with a different name. Once install is complete you'll navigate to localhost/<your web alias> and you should be able to access the Innovator server.
AJ