No, it's not a problem and should probably be logged at a lower level.
To make it go away you need to edit your cluster-service.xml file, tc5-cluster-service.xml file and any other JBoss Cache config file you're using. Find the UNICAST element and remove the window_size and min_threshold attributes.
The reason is that you're using a version of JGroups which is newer than the version of the config file, and has these properties deprecated. As Brian mentioned, simply remove the properties from the config files.
You can find out the version of JGroups by executing:
java -cp jgroups.jar org.jgroups.Version