You may need to set this to true in user.config then touch Web.config to make it reload settings.
<add key="UseLegacyCryptoHelper" value="true" />
but this is not related to a version upgrade from 2.3.6.4 to 2.3.6.5, maybe if you changed to using a user.config file and you were previously only using Web.config, it might bring this as a new problem.
Hope it helps,
Joe