Do You know what's wrong? When enabled mongodb_session I have errors from views and context modules, only these 2 modules I have enabled:

    Notice: Undefined property: stdClass::$roles in context_condition_user->execute() (line 24 of /home/../sites/all/modules/context/plugins/context_condition_user.inc).
    Warning: Inv.alid argument supplied for foreach() in context_condition_user->execute() (line 25 of /home/.../sites/all/modules/context/plugins/context_condition_user.inc).
    Notice: Undefined property: stdClass::$roles in views_plugin_cache->get_results_key() (line 267 of /home/.../sites/all/modules/views/plugins/views_plugin_cache.inc).
    Warning: array_keys() expects parameter 1 to be array, null given in views_plugin_cache->get_results_key() (line 267 of /home/.../sites/all/modules/views/plugins/views_plugin_cache.inc).
    Notice: Undefined property: stdClass::$roles in views_plugin_cache->get_output_key() (line 288 of /home/.../sites/all/modules/views/plugins/views_plugin_cache.inc).
    Warning: array_keys() expects parameter 1 to be array, null given in views_plugin_cache->get_output_key() (line 288 of /home/.../sites/all/modules/views/plugins/views_plugin_cache.inc).

Comments

Looks like a compatibility issue between those modules. I wouldn't have to replicate it to investigate. I'm working a somewhat similar problem with stdClass::$uid is undefined in an SSL switching website.

In mongo_session.inc I find comment:

Add roles element to $user and insert an entry into fields_current.user so next time we get it from Mongo.

but the roles do not saving into database, so I try add manually:
> db.fields_current.user.update({"_id":1},{$set : {"roles" : {0:'newbie'}}})
and the error was gone, even the context module responds correctly.