activiti-spring POM contains dependency on slf4j-log4j12. That should not be there, the choice of SLF4J binding is up to the application developer or even deployment, not library developer.
If there is any other binding on the classpath (such as logback), SLF4J complains and (in my case) still chooses the wrong one (log4j).
It is possible to workaround this by excluding the dependency in the POM like this:
That is obviously only a workaround and not a permanent fix.