Well it is difficult to know without having the stack trace. It's possible
that a class in the model:
   a) is not Serializable
   b) is not found in the classpath
   c) is a different version to the class that was used to create the
original serialized data (it has been modified and hence the serial numbers
of the classes don't match)

As for why it's only occurring sometimes, who knows? Maybe the particular
piece of code isn't always called, or the object model may not always
contain a reference to the troublesome class. It could also be a standard IO
error like corrupted stream

Is there no way of getting at the stack trace? You've checked the server
logs?