Installation instructions request that slf4j-api JAR file is copied to Tomcats lib directory. And correctly also as the Tomcat won't start if it is not in there.
But then this API conflicts with the same API in kie-server-6.3.0.CR2-ee7.

I did a quick fix that seemed to solve this:

1. Removed kie-server WAR file from tomcat webapps directory, so that it won't undo the next step.
2. Deleted the problem file webapps/kie-server-6.3.0.CR2-ee7/WEB-INF/lib/slf4j-api-1.7.2.jar

I considered removing the jar file from Drools project, but that is likely to cause other errors, so I decided to leave this to the experts (= you guys).