Hi Steffi,
In the error you are getting this is part of that
There is no email address found to send this email to.
at EmailTemplate.SendEmailMessage (JavaAction : 'Send Email')
at EmailTemplate.IVK_SendEmail (SubMicroflow : 'SendEmailMessage')
at EmailTemplateCustomizations.ACT_DMY_FeedbackEmail (SubMicroflow : 'IVK_SendEmail')
And it says that there is no email address found.
The java action Send Email expects an email address which is probably left empty when this part of the flow is triggered.
As the emails are triggered from the ACT_DMY_FeedbackEmail microflow I would advise to place a breakpoint there and then see if and why the to address is empty.
Hope this helps.
KR, Jurre