After you define an Oracle search in the AdminConsole > Search Configuration, none of the existing or new Consumer Portal instances start anymore.
If you open a Network tab, of the web browser's developer console and refresh the page, you can observe initial requests including those pointing to a service called Backend, e.g.:
That is first and basic request sent to the Backend service, which creates a cookie in the user's browser cache. When you try to open this request directly through browser's address bar, i.e. http://localhost/backend/v2/connection/setcookie, there will be most likely more detailed error message with following or similar text:
... The type initializer for 'Oracle.DataAcces.Client.OracleConnection' threw an exception ...
To fix this issue, make sure that you have Oracle Client 32bit installed together with ODP.NET / ODAC components. Supported and tested client versions are (as of 2018 release):