Hey Dirk, thanks for reporting this issue. It is possible to have a calculation field as the trigger for an automation, we will investigate why this is currently not working.
However, if you do NOT want a calculation field to trigger you automation, you can simply use the “has changed” filter to specify the fields which should trigger your automation
Thanks for the quick solution, I just checked and I am happy to report that calculation field changes trigger automations as expected, just to make sure