Login Engine, user object "marshal data too short"?

This suddenly started happening to my app today. No code changes, so I
expect that this has something to do with a server change.

I'm using Login Engine and ActiveRecord sessions. LE stores the whole
user object in the session, then pulls it out as needed with

Out of the blue today this starts failing with the infamous "you're
serializing AR objects into the db so you're getting a 'marshal data
too short' error". No code has changed in this app in weeks and
suddenly I'm getting this error.

I can clear all sessions from the DB, do something to create a session
and store a user, then hit another page that pulls the user out of the
DB and I get a Application Error with the log saying "marshal data too