NPE in abstract node

We see a NPE in the logs after a save of a custom node. Could be that we are doing something strange but we do not know what that could be. Please advice. We save something on one node and the other node shows this behavior. But it starts working again after a restart.

at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:653)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:920)
at java.lang.Thread.run(Thread.java:724) [rt.jar:1.7.0_25]
Caused by: java.lang.NullPointerException
at org.modeshape.jcr.AbstractJcrNode.isNodeType(AbstractJcrNode.java:2376) [modeshape-jcr-3.7.1.Final.jar:3.7.1.Final]
at org.modeshape.jcr.AbstractJcrNode.isShareable(AbstractJcrNode.java:563) [modeshape-jcr-3.7.1.Final.jar:3.7.1.Final]
at org.modeshape.jcr.JcrSession.node(JcrSession.java:628) [modeshape-jcr-3.7.1.Final.jar:3.7.1.Final]
at org.modeshape.jcr.AbstractJcrNode.getNode(AbstractJcrNode.java:822) [modeshape-jcr-3.7.1.Final.jar:3.7.1.Final]
at org.modeshape.jcr.AbstractJcrNode.getNode(AbstractJcrNode.java:768) [modeshape-jcr-3.7.1.Final.jar:3.7.1.Final]
at org.modeshape.jcr.AbstractJcrNode.getNode(AbstractJcrNode.java:108) [modeshape-jcr-3.7.1.Final.jar:3.7.1.Final]