I have an older iMac running High Sierra 10.13.6. Recently the Apple Calendar app is displaying unusable subscription connections. The Calendar sidebar is shown in the image below.
In the image above, these subscription connections all contain a warning. This warning message is shown below.
I have over a hundred of these. My question: is there any way to unsubscribe from all these instead of doing it one at a time, and does anyone have a clue as to what caused all these?