For what it's worth, I had similar symptoms, but a different- self inflicted- cause. I had acquired over the years two Apple ID's, one associated with a .mac address that we had used as a "family" email, another (based on our company domain/ email) used for personal and business. Since having two Apple ID's has become problematic to say the least, when I upgraded to a new Mac and phone I tried to consolidate and use just one for all iCloud functions, leaving the other .mac address just for email. I was able to do this fine for everything except Calendar. The only "fix" I found was to disable the legacy iCloud account in Calendar preferences, and re-enter everything under the iCloud account I was using as my Main one. iCloud can only see one Apple ID, and I had my calendar data somewhere it could not look.