MGNLUI-2477 for details. But I'm pretty sure there's more classes affected. Fix for the issue should include change in pom or hudson build for UI to systematically check and ensure all ui classes that might eventually end up in session are serializable. Otherwise this bug will resurface soon again and again.
The results of the time boxed investigation are summarised at https://wiki.magnolia-cms.com/display/DEV/Serialization+issues
Based on the results of the investigation done in the past few days it has been decided to postpone support for session serialization after Magnolia 5.5. Magnolia Tomcat bundle will come with session persistence disabled as of versions 5.4.8 and 5.5.
Documentation will follow on how to disable it for other containers (