Now I have simplified my process (instead of many incoming connection i use a gateway).
The error "This type of node cannot have more than one incoming connection" does NOT occur anymore. :)
Now I'm still fighting with another error, which is the following:
ERROR builder.impl.KnowledgeBuilderImpl.newKnowledgeBase - ProcessLoadError: unable to parse xml : Exception class java.lang.NullPointerException : null
Could somebody give me a quick hint?