dwr.xml does not have an allow entry that enables conversion of this type to Javascript. The most common cause of this problem is that XXX is a java bean and bean marshalling has not been enabled. Bean marshalling is disabled by default for security reasons.
To enable marshalling for a given bean add the following line to the allow section of your dwr.xml file:
<convert converter="bean" match="XXX"/>
It is also possible to enable marshalling for an entire package or hierachy of packages using the following:
<convert converter="bean" match="package.name.*"/>
Javascript does not support overloaded methods, so the javascript file generated from this class will contain to methods the second of which will replace the first. This is probably not what you wanted.
It is best to avoid overloaded methods when using DWR.