I don’t think the ports did have something to do with it because when if one direction (as #1) works that ensures that the connection was setup correctly. Maybe you just didn’t choose the right calendar destination when you created an event?
Anyway good that you solved it
Same problem here, trying to fix it, since several days and nothing worked, hope Xiaomi is going to fix this bug quickly.
Ive written a support ticket to Xiaomi via the internal option for feedback in the settings.
Maybe you also should do that, that it gets a higher priority.