issue with rpc when behind a reverse proxy

(this is a repost, I had to change some content)

I have the following problem:
- I have a web application installed on a customer's server and running with jetty at /ksatweb. The application module is named kweb.
- the application is available on the web through a reverse proxy and mapped to https://website.com/ordering/ksatweb/

The issue I get is when using rpc. Below is the error message I get. I tried many work arounds, found by googling the issue but none worked, eg I have no control over the proxy configuration so the use of headres is not possible, etc... If anybody has an idea how to get me out of there, I am ready to do some dirty hack to get this to work...

Thanks in advance,

2012-06-27 16:16:11.752:INFO:/ksatweb:

ResourcesService: ERROR: The module path requested, /ordering/ksatweb/kweb/, is not in the same web application as this servlet, /ksatweb.  Your module may not be properly configured or your client and server code maybe out of date.
2012-06-27 16:16:11.752:INFO:/ksatweb:ResourcesService: WARNING: Failed to get the SerializationPolicy '2225236803D085D4C928146663661808' for module 'https://website.com/ordering/ksatweb/kweb/'; a legacy, 1.3.3 compatible, serialization policy will be used.  You may experience SerializationExceptions as a result.
2012-06-27 16:16:11.768:WARN:/ksatweb:Exception while dispatching incoming RPC call com.google.gwt.user.client.rpc.SerializationException: Type 'com.metaaps.webapps.ksatweb.shared.entity.User' was not assignable to 'com.google.gwt.user.client.r pc.IsSerializable' and did not have a custom field serializer.For security purposes, this type will not be serialized.: instance = com.metaaps.webapps.ksatweb.s hared.entity.User@371d6be8

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