User cannot be serialized/deserialized because it has fields that are not serializable - namely the various OSUser provider implementations have instance variables of Loggers.
Under JIRA v4.0 there is also an unserializable Concurrent Map used for caching.

Seraph puts the User object in a Session attribute.
When Weblogic tries to deserialise the Session attributes, it fails on the User stored in "seraph_defaultauthenticator_user".
WLS restores the other attirbutes and sets "seraph_defaultauthenticator_user" to null.
To Seraph, this means that the User is no longer logged in.