Thank you for insisting!! I now found the menu entry - it was not easy (it is also quite unintuitive that it is even required to make special steps to archive this). I was not aware that the calendar and the tasks are somehow the same (I was not looking for calendar at all until your hint) Here are the 8 necessary steps:
Settings
Accounts
Murena.io accout
My Account
Settings
(chose the account from a list with a single item in the Account Manager)
Calendar
tip on the three dots and select “Kalender neu erkennen” (recgonize new calendar?)
Someone else had the same or very simular issue: