Same message appears in module MendixSSO in ‘Decrypt’ using entity ‘Token’. Check if the message is coming from that module.
Check the page having snippet TokenOverviewSnippet. That snippet does not show the token-value itself though, probably because of security, but it is in the search-criteria.
O, it is totally different. The “encryption key” is fine, but the value the you pass into “Decrypt” is empty. And that value is the EmailSettings/Password. So, go back to the page for your email-settings and set the password.