Hi Franz,
Indeed the main change of this 3.0 release was to fix HTTPs support. In previous version there was an hardcoded "trust all certs" flag that:
1) was a big security hole (what the point to use HTTPs if you don't check certificate?)
2) was preventing people to use client certificates for server side authentication
So here it is very likely your TLS configuration has some issue (maybe a missing certificate in your JVM truststore). You can get more details by enabling JVM net logs:
mvn sonar:sonar -Djavax.net.debug=all
++
Julien