iCloud one-way synchronisation issue

I was finally able to resolve an iCloud synchronisation issue between my iPhone and Mac.

Calendar entries created on my Mac were immediately copied to my iPhone and iPad but entries created or edited on my iPhone were never synchronised back to the other devices. This was inconvenient because I carry my phone everywhere and needed events created there to be available on all my devices.

The solution was simple: sign out from iCloud on the phone and sign back in again.