SerializationException While Upgrading to GWT 2.8.2

I have updated from GWT 2.6.1 to GWT 2.8.2 but while running I am getting the following error in the browser:

com.google.gwt.user.client.rpc.IncompatibleRemoteServiceException: The response could not be deserialized

at Unknown.Throwable_1(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.Exception_1(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.RuntimeException_1(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.IncompatibleRemoteServiceException_1(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.onResponseReceived(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.$fireOnResponseReceived(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.onReadyStateChange(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.onreadystatechange<(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.apply_0(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.entry0_0(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.entry_0/<(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.anonymous(Unknown)

Caused by: com.google.gwt.user.client.rpc.SerializationException: 153221

at Unknown.Throwable_0(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.Exception_0(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.SerializationException_0(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.$check(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.$instantiate(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.$readObject(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.deserialize_337(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.deserialize_335(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.$deserialize_0(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.$deserialize(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.$readObject(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.read_8(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.onResponseReceived(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.$fireOnResponseReceived(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.onReadyStateChange(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.onreadystatechange<(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.apply_0(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.entry0_0(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.entry_0/<(com.ptc.windchill.wncgwt.WncGWT-0.js)

at Unknown.anonymous(Unknown)

What is the meaning of "153221" after the Error?

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