Following are the things to remember so that StateServer Mode works properly :-
√ StateServer mode session data is stored in a different process so you must ensure that your objects are serializable.
√
elements in Web.config should be indentical across all servers.So this ensures that encryption format is same across all computers.
√ IIS metabase (\LM\W3SVC\2) must be identical across all servers in that farm.