Errors in the logs for the SecurityCacheSize setting being to small or the SMP Diag reporting unused Accounts and Roles
Remove any Accounts and Roles that are not being used and adjust the SecurityCacheSize setting located in the Coresettings.config file to accommodate a larger number of entries based on the number of Accounts that exist.
Increase the Security and Item Cache settings for the SMP.
In the Notification Server Console, go to Settings> Security> Account Management. Remove any duplicates and Accounts\Roles that are no longer used.