Hi Carolin,
In addition to the links you mentioned, you can also see these highlighted sections in the image below, which only work in Marketing Emails.
Marketing Email and email in real time marketing are actually two different entities, as can be seen from their entity names.
Marketing Email: msdyncrm_marketingemail
Email in Real Time Marketing: msdynmkt_email
So if you'd like to insert the calendar into the latter one, you need to adjust the link into a fixed one.
It is recommended to send the calendar with marketing email first and copy the link from the received email.
