Ok, here is a bit of follow up:
If you have a plain WO 5.4 app, no Wonder, no nothing.
When Application.createRequest(String method, String aurl,String anHTTPVersion, Map someHeaders, NSData content, Map someInfo) is called the instantiated class type of "someHeaders" is TreeMap. If you add ERExtensions to the classpath and make no other changes, the class type passed to createRequest changes to "WOHTTPHeadersDictionary", which is what causes the WOMessage.setHeaders to spit the dummy (as per the above stack trace), because it expects the key object to be a String (as now defined in the method definition).
This anomaly is caused by the com.webobjects.appserver.WOHttpIO class that is included with wonder. As per Anjo's previous comments, you need to remove it from ERExtensions or bad things will ensue. Mike, feel like messing with the wonder builds some more?
Incidentally it appears that in WO53 createRequest() used to receive a WOHTTPHeadersDictionary, this has changed in WO54.
So to fix the problem, for now, you need to create a custom ERExtensions without WOHttpIO in it.
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Seeya...Q
Quinton Dolan - email@hidden
Gold Coast, QLD, Australia (GMT+10)
Ph: +61 419 729 806