we are haunted by occasional occurences of exceptions such as:
com.google.gwt.user.client.rpc.SerializationException: Type \'xxx\' was not assignable
I was getting a SerializationException also but I was also seeing this error showing up right before the serialization exception:
[uptimereports/2.340102563369350884].: Example : error : cannot find template registration-confirmation.vm
It turned out to be a problem finding my velocity template. Once I fixed that problem the SerializationException stopped showing up, so if you follow Kerem's advice and still have problems, look for other exceptions in your log.