Seems to be a new problem in version 3.9.x due to changed method signature.
Class
org.jivesoftware.openfire.group.Group
In version 3.8.x the method signature was
public Map<String,String> getProperties();
Now in version 3.9.x it looks like
public PersistableMap<String,String> getProperties();
So a recompile is necessary and should solve the problem.