tl/dr: Check you config-keys! 😉
I found two errors in my sitecore logs. There were keywords inside like “Alias PII removed” and “key – cannot be null”.
MessageTaskRunner worker thread 6 10:45:51 ERROR Failed to create DispatchFailedTask for 'Alias PII removed'. Exception: System.ArgumentNullException Message: Value cannot be null. Parameter name: key Source: System.Core at System.Security.Cryptography.AesManaged.CreateEncryptor(Byte key, Byte iv) at Sitecore.Modules.EmailCampaign.Core.Crypto.AuthenticatedAesStringCipher.EncryptBytes(Byte messageBytes) at Sitecore.Modules.EmailCampaign.Core.Crypto.AuthenticatedAesStringCipher.Encrypt(String message) at Sitecore.Modules.EmailCampaign.Core.Pipelines.HandleMessageEventBase.EventDataService.EmailEventToDto(EmailEvent emailEvent) at Sitecore.Modules.EmailCampaign.Core.Pipelines.HandleMessageEventBase.EventDataService.EventDataToDto(EventData eventData) at Sitecore.EmailCampaign.Cm.Dispatch.DispatchTask.CreateDispatchFailedTask(ContactIdentifier contactIdentifier, String failureReason, Boolean updateUndeliveredCount)
And another similar one:
MessageTaskRunner worker thread 11 15:24:10 ERROR Message sending error: System.ArgumentNullException: Value cannot be null. Parameter name: key at System.Security.Cryptography.AesManaged.CreateEncryptor(Byte key, Byte iv) at Sitecore.Modules.EmailCampaign.Core.Crypto.AuthenticatedAesStringCipher.EncryptBytes(Byte messageBytes) [...]
I figured that the only related thing to “encryption” were two keys in my sitecore config called:
(Also have a look here: https://doc.sitecore.com/xp/en/developers/exm/103/email-experience-manager/configure-exm-in-a-scaled-environment.html)
But they were filled (and on all systems equally – so I skipped this thought)
After some reading I realized that it said “hexadecimal 64-character string” should be in there … so actually a key that is then used to encrypt data like lists and emails, but someone put in there a generated “secure password” with lots of various characters … so the simple solution was: “the keys were in the wrong format”.