Apple Calendar (Suddenly) Not Working

My Apple Calendar was working perfectly two days ago. But today, that's not the case. I use Mojave 10.14.6 on one of our iMacs. I have an iPhone 8 Plus running the latest iOS version. We also have another iMac running the latest version of Catalina 10.15.7.

As of today, when I enter a new event on our iMac running Mojave, the event appears briefly for about 2 to 3 seconds and then completely disappears (no matter which calendar is used, happens every time). The event is not seen on any of the other devices. But when I enter a new event on our iMac running Catalina, the event stays in the calendar and also appears on our iPhone 8 Plus but does not show up on the iMac running Mojave. When I enter a new event on our iPhone 8 Plus, the event stays in the calendar and also appears on our iMac running Catalina but does not show up on the iMac running Mojave. So it's not a problem with our account.

I remember this happening a number of years back when we were still running Snow Leopard. One day, all calendars stopped working and we were pretty much forced to upgrade. Both hardware and software. I do not want to upgrade our iMac running Mojave just yet.

Is anyone else getting this? Has Apple phased out calendar support on Mojave?