It sounds like you are trying to load a MyUser entity from a session
without importing the MyUser model first.
To ensure that MyUser is available when the session middleware runs,
you must import the MyUser model in your main.py script (assuming the
usual GAE project setup). Otherwise, if a fresh instance of your app
is spun up by GAE, then it might not have imported the MyUser model
before the sessions library tries to convert a protobuf representing a
MyUser entity into an actual Python MyUser instance (leading to the
KindError you're experiencing).
~ David