[ovirt-users] NullPointerException when changing compatibility version to 4.0

Hi, i currently have a problem with changing one of our clusters to compatibility version 4.0. The Log shows a NullPointerException after several successful vms: 2017-07-19 11:19:45,886 ERROR [org.ovirt.engine.core.bll.UpdateVmCommand] (default task-31) [1acd2990] Error during ValidateFailure.: java.lang.NullPointerException at org.ovirt.engine.core.bll.UpdateVmCommand.validate(UpdateVmCommand.java:632) [bll.jar:] at org.ovirt.engine.core.bll.CommandBase.internalValidate(CommandBase.java:886) [bll.jar:] at org.ovirt.engine.core.bll.CommandBase.executeAction(CommandBase.java:391) [bll.jar:] at org.ovirt.engine.core.bll.Backend.runAction(Backend.java:493) [bll.jar:] ..... On other Clusters with the exect same configuration the change to 4.0 was successfull without a problem. Turning off the cluster for the change is also not possible because of >1200 Vms running on it. Does anyone have an idea what to do, or that to look for? Thanks Marcel