Hi @Nino,
Welcome to SonarSource community! 
Are you behind a corporate proxy? You may need to add proxy config as discussed here: Advanced Topics (scroll down to Using SonarScanner for .NET with a Proxy).
It will look something like this: SONAR_SCANNER_OPTS=-Dhttps.proxyHost=xxx -Dhttps.proxyPort=xxxx
See also: Run SonarCloud Analysis through corporate proxy
Give that a try and see how that works.
Joe