Hi @Jasperow , thanks for your message!
This could be caused by a few different reasons. My main guess would be that your account is requiring mandatory authentication for all your recipients. Try checking this information in Admin > Security Settings > Recipient Authentication Settings. My suggestion is to keep the option "The sender can require that a recipient must authenticate on any envelope sent from this account" selected, as this will make it optional for senders.
If this is not the case, check your templates directly. It is likely that when they were created, the Recipient Authentication option was requiring some type of authentication.
I hope this helps!