The problem is that if you have a process with a script as resource and this resource is not referenced as deployment resource, then the engine tries to load the script from its classpath but can't find it.
The reason for this problem is that the engine can't resolve the script from the classpath of the ProcessApplication when it parses the bpmn.xml.
A Workaround is to annotate the script as a deployment resource inside the processes.xml and in the camunda:resource-attribute.
Needs to be discussed if the workaround should be the default way or if there should be a way for the engine to look into the classpath of the process application.
Problem was found on JBoss AS7 with camunda BPM 7.2.0-SNAPSHOT and the attached war.